单线程

BACK_LOG 影响的accept队列大小

客户端把数据放到accept队列，JVM再从accept队列取。

如果accept队列太小，JVM取数据不够快，那么accept队列就会溢出

public class tomcat {
    public static final String SEPARATOR = "\r\n";
    //源码注释：requested maximum length of the queue of incoming connections.
    //BACK_LOG影响的accept队列大小
    public static final int BACK_LOG = 1024;

    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = null;
        Socket socket = null;
        OutputStream outputStream = null;
        BufferedWriter bufferedWriter = null;
        try {
            serverSocket = new ServerSocket(10000, BACK_LOG);
            System.out.println("启动服务器");
            for (; ; ) {
                //阻塞式监听，会一直等待客户端连接
                socket = serverSocket.accept();
                //读取客户端消息
                outputStream = socket.getOutputStream();
                OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputStream,"UTF-8");
                bufferedWriter = new BufferedWriter(outputStreamWriter);
                bufferedWriter.write(HttpRequest());
                bufferedWriter.flush();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                outputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                socket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                serverSocket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public static String HttpRequest() {
        String str = "<h1>tomcat</h1>";
        StringBuilder builder = new StringBuilder();
        builder.append("HTTP/1.1 200 OK").append(SEPARATOR);
        builder.append("Connection: Close").append(SEPARATOR);
        builder.append("Content-Type: text/html;charset=utf-8").append(SEPARATOR);
        builder.append("Content-Length: " + str.length()).append(SEPARATOR);
        builder.append(SEPARATOR);
        builder.append(str);
        return builder.toString();
    }
}

多线程

//采用BIO+线程池
public class tomcatThread {
    public static final String SEPARATOR = "\r\n";
    //源码注释：requested maximum length of the queue of incoming connections.
    //BACK_LOG影响的accept队列大小
    public static final int BACK_LOG = 1024;

    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(10000, BACK_LOG);
        ThreadPoolExecutor threadPoolExecutor = buildThreadPoolExecutor();
        System.out.println("启动服务器");
        for (; ; ) {
            //阻塞式监听，会一直等待客户端连接
            Socket socket = serverSocket.accept();
            threadPoolExecutor.execute(() -> {
                OutputStream outputStream = null;
                BufferedWriter bufferedWriter = null;
                try {
                    //读取客户端消息
                    outputStream = socket.getOutputStream();
                    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputStream, "UTF-8");
                    bufferedWriter = new BufferedWriter(outputStreamWriter);
                    //模拟在做事
                    Thread.sleep(500);
                    bufferedWriter.write(HttpRequest());
                    bufferedWriter.flush();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            });
        }
    }

    public static String HttpRequest() {
        String str = "<h1>tomcat</h1>";
        StringBuilder builder = new StringBuilder();
        builder.append("HTTP/1.1 200 OK").append(SEPARATOR);
        builder.append("Connection: Close").append(SEPARATOR);
        builder.append("Content-Type: text/html;charset=utf-8").append(SEPARATOR);
        builder.append("Content-Length: " + str.length()).append(SEPARATOR);
        builder.append(SEPARATOR);
        builder.append(str);
        return builder.toString();
    }

    //线程池
    public static ThreadPoolExecutor buildThreadPoolExecutor() {
        return new ThreadPoolExecutor(
                100,
                100,
                0,
                TimeUnit.SECONDS,
                new SynchronousQueue<>(),
                new ThreadPoolExecutor.CallerRunsPolicy()
        );
    }
}