spring bean scope简要说明，有代码示例

以前一直只知道spring bean有做用域，没有怎么关注具体内容，今天特地看了，记录过程以做备忘。

做用域（5类）

做用域总计5种：singleton, prototype, request, session, global session
其中singleton, prototype为常规bean中均可以使用，默认为singleton；request, session, global session为web中特有，主要配置在如controller,action中具体配置方式有多种，此例中以springboot为示例

单例模式（singleton）

spring 容器中只存在一个具体实例，如
定义UserServiceImpl

@Service
public class UserServiceImpl implements IUserService {}

@RestController
@RequestMapping("/user")
public class UserController {
    private static final Logger logger = LoggerFactory.getLogger(UserController.class);
    @Autowired
    private IUserService userService;

    @RequestMapping("/{id}")
    @ResponseBody
    public User get(@PathVariable(required = false) Long id) {
        logger.info("userService:{}", userService.toString());
        return userService.get(1l);
    }
}

@RestController
@RequestMapping("/test")
public class TestController {
    private static final Logger logger = LoggerFactory.getLogger(TestController.class);
    @Autowired
    private IUserService userService;

    @GetMapping("/user")
    public User testUser() {
        logger.info("testController:{}", this.toString());
        logger.info("userService:{}", userService.toString());
        return userService.get(1l);
    }
}

结果：在调用两个地址后，userService.toString()打印出来的结果是一致的

原型模式（prototype）

每次注入时提供一个新的实例

代码以下：
改变服务提供方 UserServiceImpl

@Service
@Scope(value = "prototype")
public class UserServiceImpl implements IUserService {}

再次调用

结果：为两个调用者生成不一样的实例，但同一个调用者只生成一个

每次调用时生成不一样的实例

可在scope中加入 proxyMode= ScopedProxyMode.TARGET_CLASS

改变服务提供方 UserServiceImpl

@Service
@Scope(value = "prototype",proxyMode= ScopedProxyMode.TARGET_CLASS)
public class UserServiceImpl implements IUserService {}

再次调用

结果：每一次请求（调用），生成了不一样的实例

2018-3-26更新开始

---

若是调用者为原型模式，即

@Scope(value = "prototype")
public class UserController {...}

则UserServiceImpl 设置为@Scope(value = "prototype")便可，无需配置proxyMode= ScopedProxyMode.TARGET_CLASS

---

2018-3-26更新结束

request

request, session, global session为web中特有，做用域大体对应HTTP中的Request, Session和Application，本例只以request为例

保持上面代码修改UserController，代码以下：

@RestController
@RequestMapping("/user")
@Scope(value = "request")
public class UserController {
...
}

再次调用

结果：每次调用会生成新的UserController 实例

@Scope(value = "request") 只能用于 controller/restcontroller 相似web组件中，如用于service中会出现以下异常：

Error creating bean with name 'userServiceImpl': Scope 'request' is not active for the current thread; consider defining a scoped proxy for this bean if you intend to refer to it from a singleton; nested exception is java.lang.IllegalStateException: No thread-bound request found: Are you referring to request attributes outside of an actual web request, or processing a request outside of the originally receiving thread? If you are actually operating within a web request and still receive this message, your code is probably running outside of DispatcherServlet/DispatcherPortlet: In this case, use RequestContextListener or RequestContextFilter to expose the current request.

另singleton的bean引用一个prototype的bean的说明

网络中有说singleton的bean不能引用prototype的bean，经实验，是能够引用，但能力有限，未能明确引用是否会有线程安全或其余问题
此处仅作引用示例，代码以下

修改UserController，代码以下：

@RestController
@RequestMapping("/user")
public class UserController {
...
}

修改

@Service
@Scope(value = "prototype",proxyMode= ScopedProxyMode.TARGET_CLASS)
public class UserServiceImpl implements IUserService {}

再次调用

结果：在singleton的bean能够引用prototype的bean