spring bean scope簡要說明，有代碼示例

以前一直只知道spring bean有做用域，沒有怎麼關注具體內容，今天特地看了，記錄過程以做備忘。

做用域（5類）

做用域總計5種：singleton, prototype, request, session, global session
其中singleton, prototype爲常規bean中均可以使用，默認爲singleton；request, session, global session爲web中特有，主要配置在如controller,action中具體配置方式有多種，此例中以springboot爲示例

單例模式（singleton）

spring 容器中只存在一個具體實例，如
定義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);
    }
}

結果：在調用兩個地址後，userService.toString()打印出來的結果是一致的

原型模式（prototype）

每次注入時提供一個新的實例

代碼以下：
改變服務提供方 UserServiceImpl

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

再次調用

結果：爲兩個調用者生成不一樣的實例，但同一個調用者只生成一個

每次調用時生成不一樣的實例

可在scope中加入 proxyMode= ScopedProxyMode.TARGET_CLASS

改變服務提供方 UserServiceImpl

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

再次調用

結果：每一次請求（調用），生成了不一樣的實例

2018-3-26更新開始

---

若是調用者爲原型模式，即

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

則UserServiceImpl 設置爲@Scope(value = "prototype")便可，無需配置proxyMode= ScopedProxyMode.TARGET_CLASS

---

2018-3-26更新結束

request

request, session, global session爲web中特有，做用域大體對應HTTP中的Request, Session和Application，本例只以request爲例

保持上面代碼修改UserController，代碼以下：

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

再次調用

結果：每次調用會生成新的UserController 實例

@Scope(value = "request") 只能用於 controller/restcontroller 相似web組件中，如用於service中會出現以下異常：

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引用一個prototype的bean的說明

網絡中有說singleton的bean不能引用prototype的bean，經實驗，是能夠引用，但能力有限，未能明確引用是否會有線程安全或其餘問題
此處僅作引用示例，代碼以下

修改UserController，代碼以下：

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

修改

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

再次調用

結果：在singleton的bean能夠引用prototype的bean