从源码分析WebSocket的消息对象自动注入失败问题

雨点打透心脏的1/2处 2022-01-23 00:43 309阅读 0赞

最近研究websocket是发现一个问题，自动注入失败，代码如下:

@ServerEndpoint(value = "/websocket/{authToken}")
    @Component
    public class EzgoWebSocket {
        @Autowired
        private TokenService tokenService;
    }

其中的tokenService是null，然后导致用到这个对象的方法全部爆空指针异常。

研究了一波websocket源码，现在直接贴代码。

服务端与客户端建立连接时，

1.初始化连接

@Override
    public void init(WebConnection connection) {
       if (ep == null) {
          throw new IllegalStateException(
                sm.getString("wsHttpUpgradeHandler.noPreInit"));
       }
       String httpSessionId = null;
       Object session = handshakeRequest.getHttpSession();
       if (session != null ) {
          httpSessionId = ((HttpSession) session).getId();
       }
       // Need to call onOpen using the web application's class loader
       // Create the frame using the application's class loader so it can pick
       // up application specific config from the ServerContainerImpl
       Thread t = Thread.currentThread();
       ClassLoader cl = t.getContextClassLoader();
       t.setContextClassLoader(applicationClassLoader);
       try {
          wsRemoteEndpointServer = new WsRemoteEndpointImplServer(socketWrapper, webSocketContainer);
          wsSession = new WsSession(ep, wsRemoteEndpointServer,
                webSocketContainer, handshakeRequest.getRequestURI(),
                handshakeRequest.getParameterMap(),
                handshakeRequest.getQueryString(),
                handshakeRequest.getUserPrincipal(), httpSessionId,
                negotiatedExtensions, subProtocol, pathParameters, secure,
                endpointConfig);
          wsFrame = new WsFrameServer(socketWrapper, wsSession, transformation,
                applicationClassLoader);
          // WsFrame adds the necessary final transformations. Copy the
          // completed transformation chain to the remote end point.
          wsRemoteEndpointServer.setTransformation(wsFrame.getTransformation());
          //@ServerEndpoint注解标注的websocket消息对象的类名等信息都在endpointConfig
          ep.onOpen(wsSession, endpointConfig);
          webSocketContainer.registerSession(ep, wsSession);
       } catch (DeploymentException e) {
          throw new IllegalArgumentException(e);
       } finally {
          t.setContextClassLoader(cl);
       }
    }

endpointConfig对象：

![70][]

2.执行onOpen方法，在这里创建对象并执行消息对象中加了@onOpen注解的方法

@Override
    	public void onOpen(Session session, EndpointConfig endpointConfig) {
     
    		ServerEndpointConfig sec = (ServerEndpointConfig) endpointConfig;
     
    		Object pojo;
    		try {
    			//在这里创建我们自定义的消息对象
    			pojo = sec.getConfigurator().getEndpointInstance(
    					sec.getEndpointClass());
    		} catch (InstantiationException e) {
    			throw new IllegalArgumentException(sm.getString(
    					"pojoEndpointServer.getPojoInstanceFail",
    					sec.getEndpointClass().getName()), e);
    		}
    		setPojo(pojo);
    		//后面还有执行onOpen方法的代码，这里就不说了O(∩_∩)O哈哈~
    	}

3.我们进去看一下创建消息对象的过程

@Override
    	public <T> T getEndpointInstance(Class<T> clazz)
    			throws InstantiationException {
    		try {
    			return clazz.getConstructor().newInstance();
    		} catch (InstantiationException e) {
    			throw e;
    		} catch (ReflectiveOperationException e) {
    			InstantiationException ie = new InstantiationException();
    			ie.initCause(e);
    			throw ie;
    		}
    	}

这里方法很简单，就是反射new一个消息对象。

总结：

通过以上我们可以发现，websocket在连接一次就会new一个对象，由于是通过反射直接实例化对象，所有spring自动注入的bean并没什么卵用。O(∩\_∩)O哈哈~

解决方法：

@ServerEndpoint(value = "/websocket/{authToken}")
    @Component
    public class EzgoWebSocket {
     
        private static TokenService tokenService;
     
        @Autowired
        public EzgoWebSocket(TokenService tokenService){
            this.tokenService = tokenService;
        }
     
        public EzgoWebSocket(){}
    }

通过构造器注入，赋值给静态的tokenService，这样每次连接实例化消息对象时就不会是null了

注意，下面这个默认构造器一定要保留，上面的源码实例化消息对象时，调用的是默认的构造器，如果没有就会报错。

[70]: /images/20220123/05bf5be59d5449708525f1aed5f1593f.png