大多数人可能都不会使用socketTimeout，看了底层才知道一直都做错了

　　前几天一个机房网络抖动，引发了很多对外请求的超时问题，在发生问题排查日志的时候，发现了这么一个现象，httpclient我们的请求超时时间并没有按照我们的设置报超时异常

我们的大概配置如下：

RequestConfig requestConfig = RequestConfig.custom()
                .setConnectTimeout(1000)
                .setSocketTimeout(2000)
                .setConnectionRequestTimeout(1000)
                .build();

但实际却发现很多请求超时时间都达到了10几秒甚至有的二十几秒，大大超过了我们的预期时间，决定通过跟踪源码一探究竟：

原来http读取网络数据的时候是其实是使用的BufferedReader类，而我们知道java的io类其实都是对基本输入流的装饰，其底层其实是利用的SocketInputStream来读取数据，一路代码跟踪，我们跟踪到了这个方法

int read(byte b[], int off, int length, int timeout) throws IOException {
        int n = 0;

        // EOF already encountered
        if (eof) {
            return -1;
        }

        // connection reset
        if (impl.isConnectionReset()) {
            throw new SocketException("Connection reset");
        }

        // bounds check
        if (length <= 0 || off < 0 || off + length > b.length) {
            if (length == 0) {
                return 0;
            }
            throw new ArrayIndexOutOfBoundsException();
        }

        boolean gotReset = false;

        Object traceContext = IoTrace.socketReadBegin();
        // acquire file descriptor and do the read
        FileDescriptor fd = impl.acquireFD();
        try {
            n = socketRead0(fd, b, off, length, timeout);
            if (n > 0) {
                return n;
            }
        } catch (ConnectionResetException rstExc) {
            gotReset = true;
        } finally {
            impl.releaseFD();
            IoTrace.socketReadEnd(traceContext, impl.address, impl.port,
                                  timeout, n > 0 ? n : 0);
        }

        /*
         * We receive a "connection reset" but there may be bytes still
         * buffered on the socket
         */
        if (gotReset) {
            traceContext = IoTrace.socketReadBegin();
            impl.setConnectionResetPending();
            impl.acquireFD();
            try {
                n = socketRead0(fd, b, off, length, timeout);
                if (n > 0) {
                    return n;
                }
            } catch (ConnectionResetException rstExc) {
            } finally {
                impl.releaseFD();
                IoTrace.socketReadEnd(traceContext, impl.address, impl.port,
                                      timeout, n > 0 ? n : 0);
            }
        }

        /*
         * If we get here we are at EOF, the socket has been closed,
         * or the connection has been reset.
         */
        if (impl.isClosedOrPending()) {
            throw new SocketException("Socket closed");
        }
        if (impl.isConnectionResetPending()) {
            impl.setConnectionReset();
        }
        if (impl.isConnectionReset()) {
            throw new SocketException("Connection reset");
        }
        eof = true;
        return -1;
    }

这个方法的核心其实就是 socketRead0(fd, b, off, length, timeout)这个方法的调用，而这个方法是这样的：

private native int socketRead0(FileDescriptor fd,
                                   byte b[], int off, int len,
                                   int timeout)
        throws IOException;

这个是native方法，通过下载openjdk1.8源码，我们在openjdk\jdk\src\solaris\native\java\net的目录下找到了相关实现，在SocketInputStream.c文件里，代码如下：

Java_java_net_SocketInputStream_socketRead0(JNIEnv *env, jobject this,
                                            jobject fdObj, jbyteArray data,
                                            jint off, jint len, jint timeout)
{
    char BUF[MAX_BUFFER_LEN];
    char *bufP;
    jint fd, nread;

    if (IS_NULL(fdObj)) {
        /* shouldn't this be a NullPointerException? -br */
        JNU_ThrowByName(env, JNU_JAVANETPKG "SocketException",
                        "Socket closed");
        return -;
    } else {
        fd = (*env)->GetIntField(env, fdObj, IO_fd_fdID);
        /* Bug 4086704 - If the Socket associated with this file descriptor
         * was closed (sysCloseFD), then the file descriptor is set to -1.
         */
        if (fd == -) {
            JNU_ThrowByName(env, "java/net/SocketException", "Socket closed");
            return -;
        }
    }

    /*
     * If the read is greater than our stack allocated buffer then
     * we allocate from the heap (up to a limit)
     */
    if (len > MAX_BUFFER_LEN) {
        if (len > MAX_HEAP_BUFFER_LEN) {
            len = MAX_HEAP_BUFFER_LEN;
        }
        bufP = (char *)malloc((size_t)len);
        if (bufP == NULL) {
            bufP = BUF;
            len = MAX_BUFFER_LEN;
        }
    } else {
        bufP = BUF;
    }

    if (timeout) {
        nread = NET_Timeout(fd, timeout);
        if (nread <= ) {
            if (nread == ) {
                JNU_ThrowByName(env, JNU_JAVANETPKG "SocketTimeoutException",
                            "Read timed out");
            } else if (nread == JVM_IO_ERR) {
                if (errno == EBADF) {
                     JNU_ThrowByName(env, JNU_JAVANETPKG "SocketException", "Socket closed");
                 } else if (errno == ENOMEM) {
                     JNU_ThrowOutOfMemoryError(env, "NET_Timeout native heap allocation failed");
                 } else {
                     NET_ThrowByNameWithLastError(env, JNU_JAVANETPKG "SocketException",
                                                  "select/poll failed");
                 }
            } else if (nread == JVM_IO_INTR) {
                JNU_ThrowByName(env, JNU_JAVAIOPKG "InterruptedIOException",
                            "Operation interrupted");
            }
            if (bufP != BUF) {
                free(bufP);
            }
            return -;
        }
    }

    nread = NET_Read(fd, bufP, len);

    if (nread <= ) {
        if (nread < ) {

            switch (errno) {
                case ECONNRESET:
                case EPIPE:
                    JNU_ThrowByName(env, "sun/net/ConnectionResetException",
                        "Connection reset");
                    break;

                case EBADF:
                    JNU_ThrowByName(env, JNU_JAVANETPKG "SocketException",
                        "Socket closed");
                    break;

                case EINTR:
                     JNU_ThrowByName(env, JNU_JAVAIOPKG "InterruptedIOException",
                           "Operation interrupted");
                     break;

                default:
                    NET_ThrowByNameWithLastError(env,
                        JNU_JAVANETPKG "SocketException", "Read failed");
            }
        }
    } else {
        (*env)->SetByteArrayRegion(env, data, off, nread, (jbyte *)bufP);
    }

    if (bufP != BUF) {
        free(bufP);
    }
    return nread;
}

通过代码我们可以知道，数据的读取是通过NET_Timeout (fd, timeout)来实现的，我们继续跟踪代码，在linux_close.c文件中，发现了NET_Timeout的实现：

int NET_Timeout(int s, long timeout) {
    long prevtime = , newtime;
    struct timeval t;
    fdEntry_t *fdEntry = getFdEntry(s);

    /*
     * Check that fd hasn't been closed.
     */
    if (fdEntry == NULL) {
        errno = EBADF;
        return -;
    }

    /*
     * Pick up current time as may need to adjust timeout
     */
    if (timeout > ) {
        gettimeofday(&t, NULL);
        prevtime = t.tv_sec *   +  t.tv_usec / ;
    }

    for(;;) {
        struct pollfd pfd;
        int rv;
        threadEntry_t self;

        /*
         * Poll the fd. If interrupted by our wakeup signal
         * errno will be set to EBADF.
         */
        pfd.fd = s;
        pfd.events = POLLIN | POLLERR;

        startOp(fdEntry, &self);
        rv = poll(&pfd, , timeout);
        endOp(fdEntry, &self);

        /*
         * If interrupted then adjust timeout. If timeout
         * has expired return 0 (indicating timeout expired).
         */
        if (rv <  && errno == EINTR) {
            if (timeout > ) {
                gettimeofday(&t, NULL);
                newtime = t.tv_sec *   +  t.tv_usec / ;
                timeout -= newtime - prevtime;
                if (timeout <= ) {
                    return ;
                }
                prevtime = newtime;
            }
        } else {
            return rv;
        }

    }
}

代码中的关键点在 poll(&pfd, 1, timeout);poll是linux中的字符设备驱动中的一个函数，作用是把当前的文件指针挂到设备内部定义的等待

这样就很好理解了，其实这个时间是我两次读取数据之间的最长阻塞时间，如果我在网络抖动的情况下，我每次2秒之内返回一部分数据，这样我就一直不会超时了，为了验证我们的理解写了test，代码如下，一个controller，用来接受http请求：

@org.springframework.stereotype.Controller
@RequestMapping("/hello")
public class Controller {
    @RequestMapping("/test")
    public void tets(HttpServletRequest request ,HttpServletResponse response) throws IOException, InterruptedException {
        System.out.println("I'm coming");
        PrintWriter writer = response.getWriter();
        while (true){
            writer.print("ha ha ha");
            writer.flush();
            Thread.sleep(2000);
            System.out.println("I'm ha ha ha");
        }
    }
}

这个代码就是每隔2s发送一条数据，循环发送，模拟网络不好的时候，收到的数据断断续续，再来一个test用来发送请求：

 @Test
    public void tetsHttpClientHttp() throws IOException {
        RequestConfig requestConfig = RequestConfig.custom()
                .setConnectTimeout(1000)
                .setSocketTimeout(3000)
                .setConnectionRequestTimeout(1000)
                .build();

        CloseableHttpClient httpClient = HttpClientBuilder.create().setDefaultRequestConfig(requestConfig).build();
        // 创建Get请求
        HttpGet httpGet = new HttpGet("http://127.0.0.1:8080/hello/test");
        CloseableHttpResponse response =httpClient.execute(httpGet);
        HttpEntity responseEntity = response.getEntity();
        if (responseEntity != null) {
            System.out.println("响应内容为:" + EntityUtils.toString(responseEntity));
        }
    }

服务端结果如下：

客户端结果如下：

程序并没有如期抛出异常，和我们预想的一样，而当我们修改socketTimeout为1000时，经验证可以抛出java.net.SocketTimeoutException: Read timed out 异常

为此，为了更准确控制时间，我们需要自己实现超时机制:

ExecutorService executor = Executors.newFixedThreadPool(1);
        Callable<String> callable = new Callable<String>() {
            @Override
            public String call() throws Exception {
                RequestConfig requestConfig = RequestConfig.custom()
                        .setConnectTimeout(1000)
                        .setSocketTimeout(3000)
                        .setConnectionRequestTimeout(1000)
                        .build();

                CloseableHttpClient httpClient = HttpClientBuilder.create().setDefaultRequestConfig(requestConfig).build();
                // 创建Get请求
                HttpGet httpGet = new HttpGet("http://127.0.0.1:8080/hello/test");
                CloseableHttpResponse response =httpClient.execute(httpGet);
                HttpEntity responseEntity = response.getEntity();
                return EntityUtils.toString(responseEntity);
            }
        };
        Future<String> future = executor.submit(callable);
        System.out.print(future.get(5,TimeUnit.SECONDS));

这样就可以避免这种情况，在请求线程超时时抛出 java.util.concurrent.TimeoutException避免长时间占住业务线程影响我们的服务,当然这只是个例子，现实我们可能还要考虑线程数，拒绝策略等情况。