Java 注解和配置

Java 的原生注解

meta annotation

@Inherited

@Inherited 是一个元注解（annotations applied to other annotations 注解其他注解的注解），也是一个标记注解，@Inherited阐述了某个被标注的类型是被继承的。 **@Inherited annotation类型是被标注过的class的子类所继承。类并不从它所实现的接口继承annotation，方法并不从它所重载的方法继承annotation。**其查找过程是：反射 API 会在查找 @Inherited 标注的注解的时候，自底向上往继承树上方查找。

@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyAnnotation {
    String value() default "Default Value";
}

@MyAnnotation(value = "Parent Class")
public class ParentClass {
    // ...
}

public class ChildClass extends ParentClass {
    // ...
}

ChildClass child = new ChildClass();
MyAnnotation annotation = ChildClass.class.getAnnotation(MyAnnotation.class);
if (annotation != null) {
    System.out.println(annotation.value()); // 输出 "Parent Class"
}

• @Inherited 仅适用于类，对方法、属性或构造函数的注解无效。
• 子类可以定义同名注解来覆盖继承的注解。在上面的例子里面，@MyAnnotation(value = “Parent Class”)在子类里应该可以重新赋值成 @MyAnnotation(value = “Child Class”)
• 接口中的注解不会被子类继承，即使注解使用了 @Inherited。
• 如果一个类实现了一个接口，那么这个类不会继承接口中定义的任何注解。

WebSocket 注解相关的争论

[jsr356-experts] Annotation inheritance: was: Re: Potpourri of smaller issues

这个争论的问题是：

• @WebSocketMessage 可被继承吗？看起来是不可以的，因为这不是一个类注解
• @WebSocketEndpoint(/“chat”) 是一个类注解，这个类注解被子类覆盖 @WebSocketEndpoint(/“superchat”) 以后。有几个 endpoint？实际上可能是只有1个（覆盖的 semantics）就是这样，但也许 chat 是一个必须存在的endpoint，不能被覆盖，那么是不是无意中覆盖它会有问题，这是不是带来了一个不能扩展但又不是 final 的类型？

其他元注解

• @Target: Describes the targets to which an annotation can be applied; this directly corresponds to the nine contexts above
• @Retention: Describes how long the annotation should be retained by the compiler
• @Inherited: Denotes that an annotation should be inherited by a subtype if applied to a supertype
• @Deprecated: Denotes that an annotation (or any other type) should no longer be used
• @Repeatable: Denotes that an annotation can be applied multiple times in the same context; i.e. a class can have the same annotation applied to it two or more times 这一种注解最有意思，但平时没有什么用例，其大部分使用场景可以被一个复合值的 values 代替。

注意，直接在注解上加入注解，实际上就产生了组合注解，会让配置集体生效，但这和继承元注解不一样。

Spring 原生的功能

xml 的模式

spring 的配置总是：

• 有固定值的 w3c 的 xsi
• beans ns 它属于 spring org 的 schema 的一个子目录
• 多个 attribute ns （它实际上是一个目录）
• 一个 attirbute schemaLocation（它是目录里面的真正方定义的地方） 它也属于 spring org 的 schema 的一个子目录，通常是一段 xsd

<!-- beans 的空间，这样我们就可以使用 beans 元素 -->
<beans xmlns="http://www.springframework.org/schema/beans"
        <!--  w3c 的 xsi，默认总有 -->
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       <!-- 可以使用 context attribute -->
       xmlns:context="http://www.springframework.org/schema/context"
       <!-- 可以使用 mvc attribute -->
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       <!-- 可以使用 aop attribute -->
       xmlns:aop="http://www.springframework.org/schema/aop"
       <!-- 具体的 attribute 元素的定义 -->
       xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context.xsd
            http://www.springframework.org/schema/mvc
            http://www.springframework.org/schema/mvc/spring-mvc.xsd
            http://www.springframework.org/schema/aop
            http://www.springframework.org/schema/aop/spring-aop.xsd">

带有 name 的注解

@Repository("movieDao")

@Bean("writer2")

@Transactional(value="txManager1")

@Configuration

@Configuration 注解本质上还是 @Component，但又不同于 @Component，详见《Spring @Configuration 和 @Component 区别
》。@Configuration 里的 @Bean 方法可以嵌套使用，而@Component 里的 @Bean 方法不可以。
它天然可以被加载，还可以触发其他 bean 的加载。
它只能被放置在类型上。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component

@Configuration
 public class AppConfig {

     @Bean
     public MyBean myBean() {
         // instantiate, configure and return bean ...
     }
 }

// Bootstrapping @Configuration classes
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
 ctx.register(AppConfig.class);
 ctx.refresh();
 MyBean myBean = ctx.getBean(MyBean.class);
 // use myBean ...

// 除了它可以被  <context:component-scan/> 扫到，还可以自己配置 ComponentScan
@Configuration
@ComponentScan(value = "com.acme.app.services",excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
 public class AppConfig {
     // various @Bean definitions ...
 }
 
 @Configuration
 // 可以定义多个源
 @PropertySource("classpath:/com/acme/app.properties")
 @PropertySource("classpath:bar.properties")
 // 甚至这样
@PropertySources({
    @PropertySource("classpath:foo.properties"),
    @PropertySource("classpath:bar.properties")
})
 public class AppConfig {

// 带有缺省值的 value，不要用对象初始值了
@Value( "${jdbc.url:aDefaultUrl}" )
private String jdbcUrl;

     @Value("${bean.name}") String beanName;

     @Bean
     public MyBean myBean() {
         return new MyBean(beanName);
     }
 }
 
@Configuration
// 等同于 <import/>，需要用 AnnotationConfigApplicationContext 来 bootstrap。
@Import(DatabaseConfig.class)
public class AppConfig {

     private final DatabaseConfig dataConfig;

     public AppConfig(DatabaseConfig dataConfig) {
         this.dataConfig = dataConfig;
     }

     @Bean
     public MyBean myBean() {
         // reference the dataSource() bean method
         return new MyBean(dataConfig.dataSource());
     }
 }
 

@Configuration
// 等同于 <import/>，需要用 AnnotationConfigApplicationContext 来 bootstrap。
@ImportResource(locations={"classpath:applicationContext.xml"})
public class XmlConfiguration {

}

@Profile("development")
 @Configuration
 public class EmbeddedDatabaseConfig {

     @Bean
     public DataSource dataSource() {
         // instantiate, configure and return embedded DataSource
     }
 }

 @Profile("production")
 @Configuration
 public class ProductionDatabaseConfig {

     @Bean
     public DataSource dataSource() {
         // instantiate, configure and return production DataSource
     }
 }
 
@Configuration
 public class ProfileDatabaseConfig {

     @Bean("dataSource")
     @Profile("development")
     public DataSource embeddedDatabase() { ... }

     @Bean("dataSource")
     @Profile("production")
     public DataSource productionDatabase() { ... }
 }

 @Configuration
 public class AppConfig {

     @Inject DataSource dataSource;

     @Bean
     public MyBean myBean() {
         return new MyBean(dataSource);
     }

     @Configuration
     static class DatabaseConfig {
         @Bean
         DataSource dataSource() {
             return new EmbeddedDatabaseBuilder().build();
         }
     }
 }

// When we put @Lazy annotation over the @Configuration class, it indicates that all the methods with @Bean annotation should be loaded lazily.
// 这个注解当然也可以和 Component 一起用。相对应的是 eager initialization。
@Lazy
@Configuration
@ComponentScan(basePackages = "com.baeldung.lazy")
public class AppConfig {
 
    @Bean
    public Region getRegion(){
        return new Region();
    }
 
    @Bean
    public Country getCountry(){
        return new Country();
    }
}

@EnableAsync

激活异步拦截器，类似美团的 mole。

参考《How To Do @Async in Spring》：

// By default, Spring will be searching for an associated thread pool definition: either a unique TaskExecutor bean in the context, or an Executor bean named "taskExecutor" otherwise. If neither of the two is resolvable, a SimpleAsyncTaskExecutor will be used to process async method invocations.
 @Configuration
 @EnableAsync
 public class AppConfig implements AsyncConfigurer {
 
    @Override
     public Executor getAsyncExecutor() {
         ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
         executor.setCorePoolSize(7);
         executor.setMaxPoolSize(42);
         executor.setQueueCapacity(11);
         executor.setThreadNamePrefix("MyExecutor-");
         executor.initialize();
         return executor;
     }

     @Override
     public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
         return new MyAsyncUncaughtExceptionHandler();
     }
     
  @Bean
     public MyAsyncBean asyncBean() {
         return new MyAsyncBean();
     }
     
 }
 
 public class MyAsyncBean {
 // First – let's go over the rules – @Async has two limitations:
// it must be applied to public methods only
// self-invocation – calling the async method from within the same class – won't work
// The reasons are simple – the method needs to be public so that it can be proxied. And self-invocation doesn't work because it bypasses the proxy and calls the underlying method directly.
// 这个注解会让 Spring 生成一个 aspect 方面
 @Async("threadPoolTaskExecutor")
// 无参数异步方法
public void asyncMethodWithConfiguredExecutor() {
    System.out.println("Execute method with configured executor - "
      + Thread.currentThread().getName());
    }

// 有返回值的方法。Future.get() 会抛出 ExecutionException。从这个异常里可以取出原有的异常-不管是受检异常还是非受检异常。
@Async
public Future<String> asyncMethodWithReturnType() {
    System.out.println("Execute method asynchronously - "
      + Thread.currentThread().getName());
    try {
        Thread.sleep(5000);
        return new AsyncResult<String>("hello world !!!!");
    } catch (InterruptedException e) {
        //
    }
 
    return null;
}
 }
 
// 自定义异常处理器，这要求在 AsyncConfigurer 里专门使用工厂方法生成相应的 bean。
public class CustomAsyncExceptionHandler
  implements AsyncUncaughtExceptionHandler {
 
    @Override
    public void handleUncaughtException(
      Throwable throwable, Method method, Object... obj) {
  
        System.out.println("Exception message - " + throwable.getMessage());
        System.out.println("Method name - " + method.getName());
        for (Object param : obj) {
            System.out.println("Parameter value - " + param);
        }
    }
}

对应的 xml

<beans>

    <task:annotation-driven executor="myExecutor" exception-handler="exceptionHandler"/>

    <task:executor id="myExecutor" pool-size="7-42" queue-capacity="11"/>

    <bean id="asyncBean" class="com.foo.MyAsyncBean"/>

    <bean id="exceptionHandler" class="com.foo.MyAsyncUncaughtExceptionHandler"/>

</beans>

@EnableScheduling

激活任务调度。

 @Configuration
 @EnableScheduling
 public class AppConfig {
    // 1.cron是设置定时执行的表达式，如 0 0/5 * * * ?每隔五分钟执行一次
    // 2.zone表示执行时间的时区
    // 3.fixedDelay 和fixedDelayString 表示一个固定延迟时间执行，上个任务完成后，延迟多长时间执行
    // 4.fixedRate 和fixedRateString表示一个固定频率执行，上个任务开始后，多长时间后开始执行
     // 5.initialDelay 和initialDelayString表示一个初始延迟时间，第一次被调用前延迟的时间
     // 每 1000 毫秒运行一次
     @Scheduled(fixedRate=1000)
     public void work() {
         // task execution logic
     }
 }
 
 
 public class MyTask {

     @Scheduled(fixedRate=1000)
     public void work() {
         // task execution logic
     }
 }
 
@Configuration
 @EnableScheduling
 public class AppConfig implements SchedulingConfigurer {

     @Override
     public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
         taskRegistrar.setScheduler(taskExecutor());
     }

     @Bean(destroyMethod="shutdown")
     public Executor taskExecutor() {
         return Executors.newScheduledThreadPool(100);
     }
 }
 
 // 自定义任务执行
 @Override
     public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
         taskRegistrar.setScheduler(taskScheduler());
         taskRegistrar.addTriggerTask(
             new Runnable() {
                 public void run() {
                     myTask().work();
                 }
             },
             new CustomTrigger()
         );
     }

     @Bean(destroyMethod="shutdown")
     public Executor taskScheduler() {
         return Executors.newScheduledThreadPool(42);
     }

     @Bean
     public MyTask myTask() {
         return new MyTask();
     }

对应的 xml

<beans xmlns="http://www.springframework.org/schema/beans"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:context="http://www.springframework.org/schema/context"
   xmlns:task="http://www.springframework.org/schema/task"
   xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context-3.2.xsd
        http://www.springframework.org/schema/task
        http://www.springframework.org/schema/task/spring-task-3.2.xsd">
    <task:annotation-driven executor="jobExecutor" scheduler="jobScheduler" />
   <task:executor id="jobExecutor" pool-size="5"/>
   <task:scheduler id="jobScheduler" pool-size="10" />
</beans>

@EnableTransactionManagement

TransactionInterceptor 是被 proxy 或者 advice 加入到调用栈中：

@Configuration
// In both of the scenarios above, @EnableTransactionManagement and <tx:annotation-driven/> are responsible for registering the necessary Spring components that power annotation-driven transaction management, such as the TransactionInterceptor and the proxy- or AspectJ-based advice that weave the interceptor into the call stack when JdbcFooRepository's @Transactional methods are invoked.
 @EnableTransactionManagement
 public class AppConfig {

     @Bean
     public FooRepository fooRepository() {
         // configure and return a class having @Transactional methods
         return new JdbcFooRepository(dataSource());
     }

     @Bean
     public DataSource dataSource() {
         // configure and return the necessary JDBC DataSource
     }

     @Bean
     public PlatformTransactionManager txManager() {
         return new DataSourceTransactionManager(dataSource());
     }
 }

<beans>
     <tx:annotation-driven/>
     <bean id="fooRepository" class="com.foo.JdbcFooRepository">
         <constructor-arg ref="dataSource"/>
     </bean>
     <bean id="dataSource" class="com.vendor.VendorDataSource"/>
     <bean id="transactionManager" class="org.sfwk...DataSourceTransactionManager">
         <constructor-arg ref="dataSource"/>
     </bean>
 </beans>

Please note that proxy mode allows for interception of calls through the proxy only; local calls within the same class cannot get intercepted that way.

Note that if the mode() is set to AdviceMode.ASPECTJ, then the value of the proxyTargetClass() attribute will be ignored. Note also that in this case the spring-aspects module JAR must be present on the classpath, with compile-time weaving or load-time weaving applying the aspect to the affected classes. There is no proxy involved in such a scenario; local calls will be intercepted as well.

aspectj 的织入可以增强本地调用，默认的 proxy mode 不可以。

AdviceMode.PROXY
AdviceMode.ASPECTJ

各种 mode 的解释见《Optimal @EnableTransactionManagement Configuration》，必须配合proxyTargetClass配置使用：

This configuration says how the transaction aspect will be applied.
Briefly:

adviceMode=proxy, proxyTargetClass=true Cglib is used as proxy
mechanism. If you use this, cglib must be on classpath, your proxied
classes must have nonparametric constructor and they can’t be final
(cglib creates a child class as the proxy).

adviceMode=proxy, proxyTargetClass=false Jdk proxy mechanism is used.
You only can proxy classes that implements a interface for methods
that should be transactional. Jdk proxy can be type casted to the
interfaces but can’t be type casted as the original proxied class.

So, for adviceMode=proxy, the decision relies more on how are your
code standards and what constraints result from used proxy mechanism.

adviceMode=aspectJ uses aspectJ library, which does byte code
intrumentation instead of proxying.

adviceMode=aspectJ, compile-time weaving You should incorporate
aspectJ instrumentation during a build process in your build scripts.

adviceMode=aspectJ, load-time weaving Instrumentation is performed on
runtime. You have to put the aspectj agent as jvm parameter.

Using aspectJ is more powerful and probably more performant. It is
also less invasive in terms of restrictions put on the classes you
want to be transactional. However, proxy mode is simple Spring’s out
of the box solution.

基本上静态织入的 AspectJ 的性能最好（我们大多数时候都习惯使用 compile-time-weaving，但其实 AspectJ 还支持 load-time-weaving），但平时我们用得最多的还是 cglib 生成的 proxy（因为 Spring 会自动帮我们决策最优的方案）。

@EnableAspectJAutoProxy

@Configuration
// 这个注解本身和 aspectj 没什么必然关系（使用它并不一定带来编译时增强），倒是没有它 @Aspect 注解不能生效
@EnableAspectJAutoProxy
public class AppConfig {

     @Bean
     public FooService fooService() {
         return new FooService();
     }

     @Bean
     public MyAspect myAspect() {
         return new MyAspect();
     }
 }


// 如果没有 @bean 工厂方法，则这里必须加上 @Component
@Aspect
public class MyAspect {
     @Before("execution(* FooService+.*(..))")
     public void advice() {
         // advise FooService methods as appropriate
     }
 }

web application context 和 DispatcherServlet application contexts 是两个 context，需要单独声明 @EnableAspectJAutoProxy at multiple levels。

@EnableWebMvc

@Configuration
@EnableWebMvc
@ComponentScan(
       basePackageClasses = { MyConfiguration.class },
       excludeFilters = { @Filter(type = FilterType.ANNOTATION, value = Configuration.class) }
)
public class MyConfiguration extends WebMvcConfigurerAdapter {

       @Override
       public void addFormatters(FormatterRegistry formatterRegistry) {
               formatterRegistry.addConverter(new MyConverter());
       }

       @Override
       public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
               converters.add(new MyHttpMessageConverter());
       }

       // @Override methods ...

}

ContextConfiguration

spring-test特有,ContextConfiguration 要和@Configuration或者@Component配合使用。

@RunWith(SpringRunner.class)
@TestPropertySource("/foo.properties")
@ContextConfiguration(classes = {AppConfig.class, DatabaseConfig.class})
@TestPropertySource(properties = {"foo=bar"})
// SpringBoot 特有
@SpringBootTest(properties = {"foo=bar"}, classes = SpringBootPropertiesTestApplication.class)
public class MyTests {

    @Autowired MyBean myBean;

    @Autowired DataSource dataSource;

    @Test
    public void test() {
        // assertions against myBean ...
    }
}

@PropertySource

property 跨上下文继承的关系见《Properties with Spring and Spring Boot》。
《官方的列表》。

@Configuration
 @PropertySource("classpath:/com/acme/app.properties")
 public class AppConfig {

     @Inject Environment env;

     @Bean
     public MyBean myBean() {
         return new MyBean(env.getProperty("bean.name"));
     }
 }

@ComponentScan

它指定的扫描属性依赖于 basePackageClasses()/basePackages() (or its alias value()。

环境 API

环境 API 意味着对 properties 和 profile 的建模，Environment接口扩展了PropertyResolver接口。

参考《Spring的Property配置加载和使用过程及Environment的初始化过程
》：

首先，PropertySource其实就是包装的具体配置，跟Properties差不多。

而PropertyResolver，就是用于对PropertySource进行特殊处理，比如解析holder、转换值的类型等。

Spring启动时，默认会new一个StandardEnvironment，这个类里面就默认添加了两个PropertySource（SystemProperties和SystemEnvironment，分别对应System.getenv和System.getProperty）

注意，可能是为了使用方便，Environment实现了PropertyResolver接口。

每一个参数，但凡可以用-D 动态传入，也应该可以使用环境变量，甚至 JNDI 的配置，其相对顺序参考《Spring Boot Configuration Priority order》：

1. command-line arguments.
2. The Java system parameters obtained through System.getproperties ().
3. Operating system environment variables.
4. The JNDI attribute obtained from java:comp/env.
5. The “random.*” property generated by Randomvaluepropertysource.
6. Apply the properties file outside of the Jar file . (via
7. spring.config.location parameter)
8. Apply the properties file inside the Jar file.
9. A property file that is declared through the “@PropertySource”
10. annotation in the application Configuration Java class (the Java
11. class that contains the “@Configuration” annotations).
12. The default property declared by the “Springapplication.setdefaultproperties”.

在 java 中获取环境变量：环境变量System.getenv() 或者 environment.getenv()

System.getenv() 方法是获取指定的环境变量的值。它有两种方法，一种是接收参数为任意字符串，当存在指定环境变量时即返回环境变量的值，否则返回null。另外一种是不接受参数，那么返回的是所有的环境变量。

// 接收参数为任意字符串
public static String getenv(String name) {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv."+name));
        }

        return ProcessEnvironment.getenv(name);
    }

// 不接受参数
public static java.util.Map<String,String> getenv() {
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("getenv.*"));
        }

        return ProcessEnvironment.getenv();
    }

在 java 中获取属性：System.getProperty() 或者 environment.getProperty()

获取系统的相关属性，包括文件编码、操作系统名称、区域、用户名等，此属性一般由jvm自动获取，不能设置。这个必须接受一个String类型的参数，返回值的类型也是String，如果想获取所有的系统的相关属性值可以使用System.getProperties（）

java.version Java 运行时环境版本
java.vendor Java 运行时环境供应商
java.vendor.url Java 供应商的 URL
java.home Java 安装目录
java.vm.specification.version Java 虚拟机规范版本
java.vm.specification.vendor Java 虚拟机规范供应商
java.vm.specification.name Java 虚拟机规范名称
java.vm.version Java 虚拟机实现版本
java.vm.vendor Java 虚拟机实现供应商
java.vm.name Java 虚拟机实现名称
java.specification.version Java 运行时环境规范版本
java.specification.vendor Java 运行时环境规范供应商
java.specification.name Java 运行时环境规范名称
java.class.version Java 类格式版本号
java.class.path Java 类路径
java.library.path 加载库时搜索的路径列表
java.io.tmpdir 默认的临时文件路径
java.compiler 要使用的 JIT 编译器的名称
java.ext.dirs 一个或多个扩展目录的路径
os.name 操作系统的名称
os.arch 操作系统的架构
os.version 操作系统的版本
file.separator 文件分隔符（在 UNIX 系统中是“/” ）
path.separator 路径分隔符（在 UNIX 系统中是“:” ）
line.separator 行分隔符（在 UNIX 系统中是“/n” ）
user.name 用户的账户名称
user.home 用户的主目录
user.dir 用户的当前工作目录

对应的命令行用法是java -jar jarName -DpropertyName=value，如java -Djavadoop.database.password=admin4321 -jar app.jar。

参考《System.getenv()和System.getProperty() 的区别》。

profile 的定义

a named, logical group of bean definitions to be registered with the
container only if the given profile is active

和 Configuration 类似，用来聚合 bean。与之相对应地，maven 中的 profile 就是用来聚合配置用的。一旦被注册进了某个 profile，则 bean 不会轻易地被默认激活。

@Component
@Profile("dev")
public class DevDatasourceConfig

@Component
@Profile("!dev")
public class DevDatasourceConfig

// 程序式地激活 profile 的方法
@Configuration
public class MyWebApplicationInitializer 
  implements WebApplicationInitializer {
 
    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
  
        servletContext.setInitParameter(
          "spring.profiles.active", "dev");
    }
}

@Autowired
private ConfigurableEnvironment env;
...
env.setActiveProfiles("someProfile");

<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>/WEB-INF/app-config.xml</param-value>
</context-param>
<context-param>
    <param-name>spring.profiles.active</param-name>
    <param-value>dev</param-value>
</context-param>

-Dspring.profiles.active=dev
export spring_profiles_active=dev

properties 的例子

properties files, JVM system properties, system environment variables,
JNDI, servlet context parameters, ad-hoc Properties objects, Maps, and
so on.

所有的 properties 都由PropertySourcesPlaceholderConfigurer对${}进行注入。

@Configuration
public class AppConfig {

    @Autowired Environment env;

    @Bean
    public MyBean myBean() {
        MyBean myBean = new MyBean();
        myBean.setName(env.getProperty("bean.name"));
        return myBean;
    }
}

Aware 接口

• ApplicationContextAware
• ApplicationEventPublisherAware
• BeanClassLoaderAware
• BeanFactoryAware
• BeanNameAware
• BootstrapContextAware
• EmbeddedValueResolverAware
• EnvironmentAware
• ImportAware
• LoadTimeWeaverAware
• MessageSourceAware
• NotificationPublisherAware
• ResourceLoaderAware
• SchedulerContextAware
• ServletConfigAware
• ServletContextAware

PropertySourcesPlaceholderConfigurer

其中PropertyPlaceholderConfigurer是Spring3.1之前使用的。
PropertySourcesPlaceholderConfigurer是Spring3.1之后使用的。

有了这个机制，才能让特定的 .properties 注入到特定的 xml 占位符里面。

<!-- bean 形式 -->
<bean id="propertyConfigurer"class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
   <property name="location">
     <value>conf/sqlmap/jdbc.properties</value>
   </property>
    <property name="fileEncoding">
      <value>UTF-8</value>
    </property>
</bean>

<bean id="propertyConfigurer"class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">  
    <property name="locations">
        <list>
            <value>/WEB-INF/mail.properties</value>  
            <value>classpath: conf/sqlmap/jdbc.properties</value>//注意这两种value值的写法
     </list>
    </property>
</bean>

<!-- spring容器中最多只能定义一个context:property-placeholder!(spring和springmvc不是同一容器，PropertyPlaceholderConfigurer可以同时存在于spring和springmvc中 -->
<!-- context attribute 形式，这种形式更优于 bean 的形式 -->
<context:property-placeholder location="classpath*:/WEB-INF/mail.properties" />

PropertySourcesPlaceholderConfigurer本质上是一个BeanFactoryPostProcessor。解析XML的流程在BeanFactoryPostProcessor之前， 优先将配置文件的路径以及名字通过Setter传入PropertySourcesPlaceholderConfigurer。

如上BeanFactoryPostProcessor的优先级又优于其余的Bean。因此可以实现在bean初始化之前的注入。

参考：

1. 《Spring PropertySourcesPlaceholderConfigurer工作原理》
2. 《Spring 常用的两种PropertyPlaceholderConfigurer》 基本还是 MergedProperties 那一套。
3. 《Spring Properties Loader》

xml 配置

annotation-config等字符串实际上指的是一个 element 的 attribute。

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans.xsd
            http://www.springframework.org/schema/mvc
            http://www.springframework.org/schema/mvc/spring-mvc.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"  profile="dev">

     <context:component-scan base-package="com.xh.spring.aop">
        <context:include-filter type="annotation" 
                 expression="org.aspectj.lang.annotation.Aspect"/>
        <context:exclude-filter type="annotation"
expression="org.springframework.stereotype.Controller" />
        <context:exclude-filter type="annotation"
expression="org.springframework.web.bind.annotation.RestController" />
    </context:component-scan>
    
    <context:annotation-config />

    <context:property-placeholder ignore-unresolvable="true" location="classpath*:/base.properties,classpath:database.properties" />

    <mvc:annotation-driven/>
    <!-- proxy-target-class属性值决定是基于接口的还是基于类的代理被创建。如果proxy-target-class 属性值被设置为true，那么基于类的代理将起作用（这时需要cglib库）。如果proxy-target-class属值被设置为false或者这个属性被省略，那么标准的JDK 基于接口的代理。高版本spring自动根据运行类有没有实现特定接口选择JDK或CGLIB代理，我们无需设置proxy-target-class属性，JDK动态代理是模拟接口实现的方式，cglib是模拟子类继承的方式，一般采用前者，因为前者效率高。后者不建议使用。-->
  <aop:config proxy-target-class="true">
        <aop:aspect id="log" ref="logHandler">
            <aop:pointcut id="printLog" expression="execution(* cn.sw.study.common.test.spring.aop.service..*(..))" />
            <aop:before method="LogBefore" pointcut-ref="printLog" />
            <aop:after method="LogAfter" pointcut-ref="printLog" />
        </aop:aspect>
    </aop:config>
    <bean class="com.acme.AppConfig"/>
    
    <!-- PropertiesFactoryBean 也可以支持 @Value：@Value("#{propBean['filePath']}")或者@Value("#{propBean.filePath}") -->
    <bean id="propBean" class="org.springframework.beans.factory.config.PropertiesFactoryBean">
  <property name="locations" value="classpath:jdbc.properties"/>  
</bean>
    
    <!-- 事务管理 -->
    <tx:annotation-driven transaction-manager="transactionManager"/>
    <bean id="transactionManager"
          class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
        <property name="dataSource">
            <ref bean="routingDatasource"/>
        </property>
    </bean>
    
    <!-- 引用其它配置文件 -->
    <import resource="a.xml"/>
    <import resource="b.xml"/>
    <import resource="springmvc-web.xml"/>
</beans>

<context:component-scan/> 
<context:property-placeholder/>
<!-- @Value("${filePath}") -->
<context:property-placeholder location="classpath*:/WEB-INF/mail.properties" />

@ResponseBody

表示该方法的返回结果直接写入HTTP response body中，一般在异步获取数据时使用，在使用@RequestMapping后，返回值通常解析为跳转路径，

加上@responsebody后返回结果不会被解析为跳转路径，而是直接写入HTTP response body中；比如异步获取json数据，加上@responsebody后，会直接返回json数据；

@RequestBody

参数前加上这个注解之后，认为该参数必填。表示接受json字符串转为对象 List等；

@Qualifier

当有多个同一类型的Bean时，可以用@Qualifier(“name”)来指定。与@Autowired配合使用

@Autowired

按照类型（byType）装配依赖对象，默认情况下它要求依赖对象必须存在，如果允许null值，可以设置它的required属性为false。如果我们想使用按照名称（byName）来装配，可以结合@Qualifier注解一起使用。(通过类型匹配找到多个candidate,在没有@Qualifier、@Primary注解的情况下，会使用对象名作为最后的fallback匹配)。

@Resource

@Resource(name=”name”,type=”type”)

默认按照ByName自动注入，由J2EE提供，需要导入包javax.annotation.Resource。@Resource有两个重要的属性：name和type，而Spring将@Resource注解的name属性解析为bean的名字，而type属性则解析为bean的类型。所以，如果使用name属性，则使用byName的自动注入策略，而使用type属性时则使用byType自动注入策略。如果既不制定name也不制定type属性，这时将通过反射机制使用byName自动注入策略。

@RequestMapping

RequestMapping是一个用来处理请求地址映射的注解，可用于类或方法上。用于类上，表示类中的所有响应请求的方法都是以该地址作为父路径；

params:指定request中必须包含某些参数值是，才让该方法处理。
headers:指定request中必须包含某些指定的header值，才能让该方法处理请求。
value:指定请求的实际地址，指定的地址可以是URI Template 模式
method:指定请求的method类型， GET、POST、PUT、DELETE等
consumes:指定处理请求的提交内容类型（Content-Type），如application/json,text/html;
produces:指定返回的内容类型，仅当request请求头中的(Accept)类型中包含该指定类型才返回。

它还有其他语法糖：@GetMapping、@PostMapping

@RequestParam

用在方法的参数前面。相当于 request.getParameter；

@PathVariable

路径变量。如 RequestMapping(“user/get/mac/{macAddress}”) ；

public String getByMacAddress(
@PathVariable(“macAddress”) String macAddress){
//do something;
}
参数与大括号里的名字相同的话，注解后括号里的内容可以不填。

@ControllerAdvice

@ControllerAdvice是一个特殊的 @Component，用于标识一个类，这个类中被以下三种注解标识的方法：@ExceptionHandler，@InitBinder，@ModelAttribute，将作用于所有的@Controller类的接口上。这三种方法，可以被认为是三种 advice。

@ControllerAdvice
public class ActionAdvice {
    // 注册属性编辑器，对HTTP请求参数进行处理，再绑定到对应的接口，比如格式化的时间转换等。应用于单个@Controller类的方法上时，仅对该类里的接口有效。与@ControllerAdvice组合使用可全局生效。
    @InitBinder
    public void handleException(WebDataBinder binder) {
        binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd HH:mm:ss"));
    }
    
    // 统一异常处理，也可以指定要处理的异常类型
    @ExceptionHandler(Exception.class)
    @ResponseBody
    @ResponseStatus(HttpStatus.OK)
    public Map handleException(Exception ex) {
        Map<String, Object> map = new HashMap<>();
        map.put("code", 400);
        map.put("msg", ex.toString());
        return map;
    }
    
    // 一个 advice 类里可以有多个 handleException 映射
}

@RestController
public class BasicController {
    
    @GetMapping(value = "index")
    public Map index(@ModelAttribute("user") String user) {
        //...
    }
}

DelegatingFilterProxy

过滤器，它指向一个bean，这个bean在spring中的名字为testBean，testBean也必需实现javax.servlet.Filter。

<filter>  
        <filter-name>testFilter</filter-name>  
        <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>  
        <init-param>  
            <param-name>targetBeanName</param-name>  
            <param-value>testBean</param-value>  
        </init-param>  
        <init-param>  
            <param-name>contextAttribute</param-name>  
            <param-value>session</param-value>  
        </init-param>  
        <init-param>  
            <param-name>targetFilterLifecycle</param-name>  
            <param-value>false</param-value>  
        </init-param>  
</filter>
        
<filter-mapping>  
    <filter-name>testFilter</filter-name>  
    <url-pattern>/*</url-pattern>  
</filter-mapping>

JPA 特性

JPA注解式配置.xmind

@Table

@Table(name=”“)
表明这是一个实体类。一般用于jpa ，这两个注解一般一块使用，但是如果表名和实体类名相同的话，@Table可以省略；

@MappedSuperClass

用在确定是父类的entity上。父类的属性子类可以继承；

@NoRepositoryBean

一般用作父类的repository，有这个注解，spring不会去实例化该repository；

@Column

如果字段名与列名相同，则可以省略；

@Id

表示该属性为主键；

@GeneratedValue

@GeneratedValue(strategy=GenerationType.SEQUENCE,generator = “repair_seq”)

表示主键生成策略是sequence（可以为Auto、IDENTITY、native等，Auto表示可在多个数据库间切换），指定sequence的名字是repair_seq；

参考《JPA 的 id 生成策略》。

@SequenceGeneretor

@SequenceGeneretor(name = “repair_seq”, sequenceName = “seq_repair”, allocationSize = 1)

name为sequence的名称，以便使用，sequenceName为数据库的 sequence 名称，两个名称可以一致；

要底层的 RDBMS 能够支持 sequence 功能。

@Transient

表示该属性并非一个到数据库表的字段的映射,ORM框架将忽略该属性.

如果一个属性并非数据库表的字段映射,就务必将其标示为@Transient,否则,ORM框架默认其注解为@Basic；

@Basic(fetch=FetchType.LAZY)

@Basic(fetch=FetchType.LAZY)

标记可以指定实体属性的加载方式；

@JsonIgnore

作用是json序列化时将java bean中的一些属性忽略掉,序列化和反序列化都受影响

@JoinColumn(name=”loginId”)

@JoinColumn(name=”loginId”)

一对一：本表中指向另一个表的外键。
一对多：另一个表指向本表的外键。

表之间的映射

对应Hibernate配置文件中的一对一，一对多，多对一。

哪一边是 owning side 很重要。

OneToOne

Implementing with a Foreign Key in JPA

@Entity
@Table(name = "users")
public class User {
     
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;
    //... 
 
    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "address_id", referencedColumnName = "id")
    private Address address;
 
    // ... getters and setters
}

@Entity
@Table(name = "address")
public class Address {
 
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    // The address side of the relationship is called the non-owning side. 
    @Column(name = "id")
    private Long id;
    //...
 
    @OneToOne(mappedBy = "address")
    private User user;
 
    //... getters and setters
}

Implementing with a Shared Primary Key in JPA

@Entity
@Table(name = "users")
public class User {
 
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;
 
    //...
 
    @OneToOne(mappedBy = "user", cascade = CascadeType.ALL)
    private Address address;
 
    //... getters and setters
}

@Entity
@Table(name = "address")
public class Address {
 
    @Id
    @Column(name = "id")
    private Long id;
 
    //...
 
    @OneToOne
    // @MapsId tells Hibernate to use the id column of address as both primary key and foreign key. Also, notice that the @Id column of the Address entity no longer uses the @GeneratedValue annotation.
    @MapsId
    private User user;
    
    //... getters and setters
}

Modeling with a Join Table

@Entity
@Table(name = "employee")
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;
 
    //...
 
    @OneToOne(cascade = CascadeType.ALL)
    @JoinTable(name = "emp_workstation", 
      joinColumns = 
        { @JoinColumn(name = "employee_id", referencedColumnName = "id") },
      inverseJoinColumns = 
        { @JoinColumn(name = "workstation_id", referencedColumnName = "id") })
    private WorkStation workStation;
 
    //... getters and setters
}

@Entity
@Table(name = "workstation")
public class WorkStation {
 
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;
 
    //...
 
    @OneToOne(mappedBy = "workStation")
    private Employee employee;
 
    //... getters and setters
}

@OneToMany

As stated in the JPA specification under section 2.9, it’s a good practice to mark many-to-one side as the owning side.

平凡解决方案

@Entity
@Table(name="CART")
public class Cart {
 
    //...
 
    @OneToMany(mappedBy="cart")
    private Set<Items> items;
     
    // getters and setters
}

@Entity
@Table(name="ITEMS")
public class Items {
     
    //...
    @ManyToOne
    @JoinColumn(name="cart_id", nullable=false)
    private Cart cart;
 
    public Items() {}
     
    // getters and setters
}

Cart as the Owning Side

public class ItemsOIO {
    //  ...
    @ManyToOne
    @JoinColumn(name = "cart_id", insertable = false, updatable = false)
    private CartOIO cart;
    //..
}
 
public class CartOIO {
    //..  
    @OneToMany
    @JoinColumn(name = "cart_id") // we need to duplicate the physical information
    private Set<ItemsOIO> items;
    //..
}

@ManyToOne

缺，来日补上

@ManyToMany

普通映射

@Entity
class Student {
 
    @Id
    Long id;
 
    @ManyToMany
    @JoinTable(
  name = "course_like", 
  joinColumns = @JoinColumn(name = "student_id"), 
  inverseJoinColumns = @JoinColumn(name = "course_id"))
    Set<Course> likedCourses;
 
    // additional properties
    // standard constructors, getters, and setters
}
 
@Entity
class Course {
 
    @Id
    Long id;
 
    @ManyToMany
    Set<Student> likes;
 
    // additional properties
    // standard constructors, getters, and setters
}

Joining Table

@Embeddable
class CourseRatingKey implements Serializable {
 
    @Column(name = "student_id")
    Long studentId;
 
    @Column(name = "course_id")
    Long courseId;
 
    // standard constructors, getters, and setters
    // hashcode and equals implementation
}

@Entity
class CourseRating {
 
    @EmbeddedId
    CourseRatingKey id;
 
    @ManyToOne
    @MapsId("student_id")
    @JoinColumn(name = "student_id")
    Student student;
 
    @ManyToOne
    @MapsId("course_id")
    @JoinColumn(name = "course_id")
    Course course;
 
    int rating;
     
    // standard constructors, getters, and setters
    
}

class Student {
 
    // ...
 
    @OneToMany(mappedBy = "student")
    Set<CourseRating> ratings;
 
    // ...
}
 
class Course {
 
    // ...
 
    @OneToMany(mappedBy = "course")
    Set<CourseRating> ratings;
 
    // ...
}
}

Spring Boot 特性

Spring Boot 在扫描类路径的时候扫到特定的包的时候，会自动激活事务管理、Spring MVC 等功能。

@SpringBootApplication

自带其他自动化配置：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {}

@EnableAutoConfiguration

打开这个配置后，可以通过扫描类路径、配置文件自动打开某些配置。
必须配合 @Configuration 使用。

TransactionAutoConfiguration

spring-boot 不需要打开 @EnableTransactionManagement。

// 一个隐藏起来的 @Configuration 也可以激活事务管理
@ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
@Configuration
@EnableTransactionManagement
protected static class TransactionManagementConfiguration {
}


@Configuration
@ConditionalOnClass(PlatformTransactionManager.class)
@AutoConfigureAfter({ JtaAutoConfiguration.class, HibernateJpaAutoConfiguration.class,
        DataSourceTransactionManagerAutoConfiguration.class, Neo4jDataAutoConfiguration.class })
@EnableConfigurationProperties(TransactionProperties.class)
public class TransactionAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public TransactionManagerCustomizers platformTransactionManagerCustomizers(
            ObjectProvider<PlatformTransactionManagerCustomizer<?>> customizers) {
        return new TransactionManagerCustomizers(customizers.orderedStream().collect(Collectors.toList()));
    }

    @Configuration
    @ConditionalOnSingleCandidate(PlatformTransactionManager.class)
    public static class TransactionTemplateConfiguration {

        private final PlatformTransactionManager transactionManager;

        public TransactionTemplateConfiguration(PlatformTransactionManager transactionManager) {
            this.transactionManager = transactionManager;
        }

        @Bean
        @ConditionalOnMissingBean(TransactionOperations.class)
        public TransactionTemplate transactionTemplate() {
            return new TransactionTemplate(this.transactionManager);
        }

    }

    @Configuration
    @ConditionalOnBean(PlatformTransactionManager.class)
    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    public static class EnableTransactionManagementConfiguration {

        @Configuration
        @EnableTransactionManagement(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false",
                matchIfMissing = false)
        public static class JdkDynamicAutoProxyConfiguration {

        }

        @Configuration
        @EnableTransactionManagement(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
                matchIfMissing = true)
        public static class CglibAutoProxyConfiguration {

        }

    }
}

org.springframework.boot.autoconfigure.condition

这个包下面的注解都带有一个元注解：

@Conditional(OnJavaCondition.class)

Spring 自己有一套条件体系：

OnJavaCondition extends SpringBootCondition
SpringBootCondition extends Condition

《自定义 condition 的例子》

这些注解可以放在 @Bean、@Component、乃至于一堆 bean （@Configuration）上。

@ConditionalOnBean

在早期版本有 bug，参考《深入Spring Boot：那些注入不了的Spring占位符（${}表达式）》。

仅仅在当前上下文中存在某个 bean 时，才会初始化一个 Bean。

@Component
@ConditionalOnBean(name="redisTemplate")
public class RedisOperBean {
  private final RedisTemplate redisTemplate;
  public RedisOperBean(RedisTemplate redisTemplate) {
      // ...
  }
}

@ConditionalOnClass

只有类路径里存在特定的 class 的时候，才会初始化一个 Bean。

@Bean
@ConditionalOnClass(DependedClz.class)
public LoadIfClzExists loadIfClzExists() {
    return new LoadIfClzExists("dependedClz");
}

@ConditionalOnCloudPlatform

只在特定的云平台是活动的时候，才会初始化一个 Bean。

CLOUD_FOUNDRY
HEROKU
KUBERNETES
SAP

@ConditionalOnExpression

当表达式为true的时候，才会初始化一个 Bean。

package com.roytuts.spring.conditional.on.expression;
import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
@ConditionalOnExpression(value = "${module.enabled} and ${module.submodule.enabled}")
class SpringConfig {
    @Bean
    public Module module() {
        return new Module();
    }
}

module.enabled=true
module.submodule.enabled=true

@ConditionalOnJava

只有遇到特定的 JVM version，才会初始化一个 Bean。

@Bean
@ConditionalOnMissingBean(name = "websocketContainerCustomizer")
@ConditionalOnJava(JavaVersion.SEVEN)
public TomcatWebSocketContainerCustomizer websocketContainerCustomizer() {
    return new TomcatWebSocketContainerCustomizer();
}

@ConditionalOnJndi

只有 jndi 路径上有特定的资源的时候，才会初始化一个 Bean。

@Configuration
@ConditionalOnJndi("java:comp/env/foo")
class OnJndiModule {
  ...
}

@ConditionalOnMissingBean

仅仅在当前上下文中不存在某个 bean 时，才会初始化一个 Bean。

@Bean
@ConditionalOnMissingBean(name = "notExistsBean")
public LoadIfBeanNotExists loadIfBeanNotExists() {
    return new LoadIfBeanNotExists("notExistsBean");
}

@ConditionalOnMissingClass

class不存在时，才会初始化一个 Bean。

@Bean
@ConditionalOnMissingClass("com.git.hui.boot.conditionbean.example.depends.clz.DependedClz")
public LoadIfClzNotExists loadIfClzNotExists() {
    return new LoadIfClzNotExists("com.git.hui.boot.conditionbean.example.depends.clz.DependedClz");
}

@ConditionalOnNotWebApplication

只有 application context 不是一个 web application context 时，才会初始化一个 Bean。

package com.roytuts.spring.conditional.on.web.notweb.application;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
@ConditionalOnWebApplication
class SpringConfigOnWebNotWebApp {
    @Bean
    public Module module() {
        return new Module();
    }
}

@ConditionalOnProperty

特定的属性有特定的值的时候，才会初始化一个 Bean。

@Configuration
//在application.properties配置"mf.assert"，对应的值为true
@ConditionalOnProperty(prefix="mf",name = "assert", havingValue = "true")
public class AssertConfig {
    @Autowired
    private HelloServiceProperties helloServiceProperties;
    @Bean
    public HelloService helloService(){
        HelloService helloService = new HelloService();
        helloService.setMsg(helloServiceProperties.getMsg());
        return helloService;
    }
}

@ConditionalOnResource

只有特定的（非 JNDI）资源存在的时候，才会初始化一个 Bean。

@ConditionalOnResource(resources = "classpath:example.json")
@Bean
ExampleService exampleService() throws Exception{
    System.out.println("Creating bean of example json from example.json...");

    File file=new ClassPathResource("example.json", this.getClass().getClassLoader()).getFile();
    return new ExampleService(new String(Files.readAllBytes(Paths.get(file.toURI()))));
}

@ConditionalOnSingleCandidate

类似 ConditionalOnBean，但要求全局只有一个 bean，或者可以决策出 primary bean 的时候，才会初始化一个 Bean。

@Bean
@ConditionalOnEnabledInfoContributor("git")
@ConditionalOnSingleCandidate(GitProperties.class)
@ConditionalOnMissingBean
@Order(DEFAULT_ORDER)
public GitInfoContributor gitInfoContributor(GitProperties gitProperties) {
    return new GitInfoContributor(gitProperties, this.properties.getGit().getMode());
}

@ConditionalOnWebApplication

和 @ConditionalOnNotWebApplication 正相反。
只有 application context 是一个 web application context 时，才会初始化一个 Bean。

@ConditionalOnWebApplication
HealthCheckController healthCheckController() {
    // ...
}

@ConditionalOnClass

只有存在一个特定的类的时候，才会初始化一个 Bean。

@Configuration
@ConditionalOnClass(DataSource.class)
class MySQLAutoconfiguration {
    //...
}

@ConfigurationProperties

Spring Boot 有个更复杂的属性加载顺序，见《2. Externalized Configuration》：

1. Devtools global settings properties in the $HOME/.config/spring-boot folder when devtools is active.
2. @TestPropertySource annotations on your tests.
3. properties attribute on your tests. Available on @SpringBootTest and the test annotations for testing a particular slice of your application.
4. Command line arguments
5. Properties from SPRING_APPLICATION_JSON (inline JSON embedded in an environment variable or system property).
6. ServletConfig init parameters.
7. ServletContext init parameters.
8. JNDI attributes from java:comp/env.
9. Java System properties (System.getProperties()).
10. OS environment variables.
11. A RandomValuePropertySource that has properties only in random.*.
12. Profile-specific application properties outside of your packaged jar (application-{profile}.properties and YAML variants).
13. Profile-specific application properties packaged inside your jar (application-{profile}.properties and YAML variants).
14. Application properties outside of your packaged jar (application.properties and YAML variants).
15. Application properties packaged inside your jar (application.properties and YAML variants).
16. @PropertySource annotations on your @Configuration classes. Please note that such property sources are not added to the Environment until the application context is being refreshed. This is too late to configure certain properties such as logging.* and spring.main.* which are read before refresh begins.
17. Default properties (specified by setting SpringApplication.setDefaultProperties).

@Configuration
@ConfigurationProperties(prefix = "javadoop.database")
public class DataBase {
String url;
String username;
String password;
// getters and setters
}

惰性加载

By default, ApplicationContext implementations eagerly create and
configure all singleton beans as part of the initialization process.
Generally, this pre-instantiation is desirable, because errors in the
configuration or surrounding environment are discovered immediately,
as opposed to hours or even days later. When this behavior is not
desirable, you can prevent pre-instantiation of a singleton bean by
marking the bean definition as lazy-initialized. A lazy-initialized
bean tells the IoC container to create a bean instance when it is
first requested, rather than at startup.

spring:
  main:
    lazy-initialization: true

随机值

# 由 RandomValuePropertySource 提供
random.number=${random.int}
random.long=${random.long}
random.uuid=${random.uuid}