1.首先用Java实现hello/hi网络聊天程序

客户端

public class TCPClient { public static void main(String[] args) throws IOException { // 创建发送端socket对象 Socket s = new Socket("127.0.0.1", 6666); // 键盘录入数据 BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); // 包装通道内的流 BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(s.getOutputStream())); // 从服务端获取返回的数据 BufferedReader brServer = new BufferedReader(new InputStreamReader(s.getInputStream())); String line = null; while ((line = br.readLine()) != null) { // 结束标记：quit if ("quit".equals(line)) { break; } // 往通道里写数据 bw.write(line); bw.newLine(); bw.flush(); // 读取服务器传送过来的数据 String lineFromServer = brServer.readLine(); System.out.println(lineFromServer); } // 释放资源 br.close(); s.close(); } }
服务器端

public class TCPServer { public static void main(String[] args) throws IOException { // 创建接收端Socket对象 ServerSocket ss = new ServerSocket(6666); // 监听客户端连接，返回一个对应的Socket对象 Socket s = ss.accept(); // 包装通道内的流 BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream())); BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(s.getOutputStream())); String line = null; while ((line = br.readLine()) != null) { System.out.println(line); // 往客户端发送数据 bw.write("服务器端已收到："+ line); bw.newLine(); bw.flush(); } //释放资源 s.close(); } }
结果
- 服务端
- 客户端

2.追踪客户端Socket

起始：第一层：TCPClient.java。

在Java中只需一行代码，客户端就创建了socket并向服务端发出连接请求（成功的话更是直接完成了与服务端的三次握手）。

// 创建发送端socket对象 Socket s = new Socket("127.0.0.1", 6666);

第二层：Socket.class

可以看到Socket的构造方法是直接调用本类的构造方法，因为传入的主机地址host!=null，所以这一层的实现为this(new InetSocketAddress(host, port))。
InetSocketAddress只是一个把主机地址和端口号封装起来的类，不是重点。因此继续跟踪this()

public Socket(String host, int port) throws UnknownHostException, IOException { this(host != null ? new InetSocketAddress(host, port) : new InetSocketAddress(InetAddress.getByName((String)null), port), (SocketAddress)null, true); }

Socket构造方法：Socket.class

把异常处理的代码去掉，可以看到try语句里的实现顺序：**this.createImpl(stream)->this.bind(localAddr)->this.connect(address)**。
分别继续深入看看三个方法各做了什么

private Socket(SocketAddress address, SocketAddress localAddr, boolean stream) throws IOException { // Socket成员变量的初始化...（省略） try { this.createImpl(stream); if (localAddr != null) { this.bind(localAddr); } this.connect(address); } // 异常处理(省略) }

3. 以this.createImpl(stream)为例的追踪过程

impl是一个SocketImpl实例。SocketImpl是一个抽象类，是实际实现套接字的所有类的公共超类。它用于创建客户端和服务器套接字。

protected abstract void create(boolean stream) throws IOException 创建流或数据报套接字。参数 stream - 如果true ，创建一个流套接字; 否则，创建一个数据报套接字。

从API得知stream如果是true则建立一个TCPsocket，false则UDPSocket

void createImpl(boolean stream) throws SocketException { if (this.impl == null) { this.setImpl(); } try { this.impl.create(stream); this.created = true; } }

继续跟进this.impl.create：AbstractPlainSocketImpl第三层

protected synchronized void create(boolean stream) throws IOException {
    this.stream = stream;
    if (!stream) {
        //略
    } else {
        this.fd = new FileDescriptor();
        this.socketCreate(true);
        SocketCleanable.register(this.fd);
    }

继续跟进this.socketCreate(true)：PlainSocketImpl第四层

native void socketCreate(boolean var1) throws IOException;
查看native方法socketCreate的源码：下载JDK源码包，找到PlainSocketImol.c,第五层 可以看到socetCreate里用C语言做了fd=JVM_Socket(domain, type, 0)

JNIEXPORT void JNICALL Java_java_net_PlainSocketImpl_socketCreate(JNIEnv env, jobject this, jboolean stream) { jobject fdObj, ssObj; int fd; //... int type = (stream ? SOCK_STREAM : SOCK_DGRAM); if ((fd = JVM_Socket(domain, type, 0)) == JVM_IO_ERR) { / note: if you run out of fds, you may not be able to load * the exception class, and get a NoClassDefFoundError * instead. / NET_ThrowNew(env, errno, "can't create socket"); return; } //... / * If this is a server socket then enable SO_REUSEADDR * automatically and set to non blocking. */ ssObj = (env)->GetObjectField(env, this, psi_serverSocketID); if (ssObj != NULL) { int arg = 1; SET_NONBLOCKING(fd); if (JVM_SetSockOpt(fd, SOL_SOCKET, SO_REUSEADDR, (char)&arg, sizeof(arg)) < 0) { NET_ThrowNew(env, errno, "cannot set SO_REUSEADDR"); close(fd); return; } } (*env)->SetIntField(env, fdObj, IO_fd_fdID, fd); }
查看JVM_Socket的实现。在jvm.cpp中找到第六层
可以看到最终最终，用到了os::socket(domain, type, protocol)。从形式上也能看出，这是调用了Linux的socket接口了，再往下就是系统调用级别了。

JVM_LEAF(jint, JVM_Socket(jint domain, jint type, jint protocol)) JVMWrapper("JVM_Socket"); return os::socket(domain, type, protocol); JVM_END

至此，经历了六层的搜索，终于追踪到了Java创建socket的接口的最底层是调用Linux的socket函数。

4. Java中各个socket方法的追踪结果总结

由于其他方法的追踪过程与创建socket的过程相似，这里直接给出各个方法的调用栈。
客户端创建socket

socket(host, port) : TCPClient.java |---this.createImpl(stream) : socket.class |---this.impl.create(stream) : socket.class |---protected synchronized void create(boolean stream) : AbstractPlainSocketImpl.class |---this.socketCreate() : AbstractPlainSocketImpl.class |---native void socketCreate(boolean var1) : PlainSocketImpl.class ------JVM------ |---Java_java_net_PlainSocketImpl_socketCreate : PlainSocketImol.c |---fd=JVM_Socket(domain, type, 0) : PlainSocketImol.c |---os::socket(domain, type, protocol) : jvm.cpp
客户端bind

socket(host,port) : TCPClient.java |---this.bind(localAddr) : socket.Class |---this.getImpl().bind(addr, port): socket.class |---protected synchronized void bind(InetAddress address, int lport) : AbstractPlainSocketImpl.class |---native void socketBind(InetAddress var1, int var2) : PlainSocketImpl.class ------JVM------ |---Java_java_net_PlainSocketImpl_socketBind : PlainSocketImol.c |---NET_Bind(int fd, struct sockaddr *him, int len) : net_util_md.c |---rv = bind(fd, him, len) : net_util_md.c |---return os::bind(fd, him, (socklen_t)len) : jvm.cpp
客户端connect

socket(host,port) : TCPClient.java |---this.connect(address) : Socket.class |---public void connect(SocketAddress endpoint, int timeout) : Socket.class |---protected void connect(SocketAddress address, int timeout) : AbstractPlainSocketImpl.class |---synchronized void doConnect(address, port, timeout): AbstractPlainSocketImpl.class |---native void socketConnect(InetAddress var1, int var2, int var3) : PlainSocketImpl.class ------JVM------ |---Java_java_net_PlainSocketImpl_socketConnect : PlainSocketImol.c |---JVM_Connect(jint fd, struct sockaddr *him, jint len) : PlainSocketImol.c |---os::connect(fd, him, (socklen_t)len) : jvm.cpp
服务端创建ServerSocket

ServerSocket(6666) : TCPServer.java |---public ServerSocket(port, backlog, bindAddr) : ServerSocket.class |---this.bind(new InetSocketAddress(bindAddr, port), backlog) : ServerSocket.class |---this.getImpl().bind(epoint.getAddress(), epoint.getPort()) : ServerSocket.class // bind上面已经跟过了 |---this.getImpl().listen(backlog) : ServerSocket.class |---this.socketListen(count) : AbstractPlainSocketImpl.class |--- native void socketListen(int var1) : PlainSocketImpl.class ------JVM------ |---Java_java_net_PlainSocketImpl_socketListen : PlainSocketImol.c |---JVM_Listen(jint fd, jint count) : PlainSocketImol.c |---os::listen(fd, count) : jvm.cpp
服务器端accept

Socket s = ss.accept() : TCPServer.class |----this.implAccept(s) : ServerSocket.class |---this.getImpl().accept(si) : ServerSocket.class |---protected void accept(SocketImpl s) : AbstractPlainSocketImpl.class |---native void socketAccept(SocketImpl var1) : PlainSocketImpl.class ------JVM------ |---Java_java_net_PlainSocketImpl_socketAccept : PlainSocketImol.c |---newfd = NET_Accept(fd, (struct sockaddr )&him, (jint)&len) : PlainSocketImol.c |---JVM_Accept(jint fd, struct sockaddr *him, jint *len) : jvm.h |---jint result = os::accept(fd, him, &socklen) : jvm.cpp