你试过了吗 printWriter.flush() 每次写入/打印后？

你的代码中有很多编译问题。某些类和对象丢失，无法解析。
但我还是试着找出问题所在。
原因可能是：
发送新行字符 \n 在 printWriter.println(commandMessage+"\n"); 声明，删除 \n .
客户机和服务器都是先写的 printWriter.println(commandMessage+"\n"); 陈述，让它在任何一堂课上都是最后的
代码如下：
studentserver.java文件：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class StudentServer {
    static ServerSocket serverSocket;
    static Socket socket;
    static PrintWriter printWriter;
    static BufferedReader bufferedReader;
    static Thread thread;

    public static void main(String args[]) throws Exception {
        StudentServer studentServer = new StudentServer();
        studentServer.serverStartActionPerformed();
        studentServer.run("server");
    }

    public void run(String commandMessage) {
        if (true) {
            try {
                printWriter.println(commandMessage);
                String input = bufferedReader.readLine();// reads the input from textfield
                System.out.println("Client message: " + input);// Append to TextArea
            } catch (Exception e) {
            }
        }
    }

    public void serverStartActionPerformed() {
        System.out.println("Server has started!");
        try {
            serverSocket = new ServerSocket(8888); // socket for the server
            socket = serverSocket.accept(); // waiting for socket to accept client
            bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream())); // reads
                                                                                                 // line
                                                                                                 // from
                                                                                                 // input
                                                                                                 // streamer
            printWriter = new PrintWriter(socket.getOutputStream(), true);
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("Server not running!"); // print message if server is not running
        }
    }
}

studentclient.java文件：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class StudentClient {
    static Socket socket;
    static PrintWriter printWriter;
    static BufferedReader bufferedReader;
    static Thread thread;

    public void clients() {
        try {
            socket = new Socket("localhost", 8888);// Socket for client
            // below line reads input from InputStreamReader
            bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            // below line writes output to OutPutStream
            printWriter = new PrintWriter(socket.getOutputStream(), true);
            System.out.println("Connected");// debug code
        } catch (Exception e) {
            System.out.println("Not Connected");
        }
    }

    public void run(String commandMessage) {
        if (true) {
            try {
                String input = bufferedReader.readLine();
                System.out.println("From server:" + input);
                printWriter.println(commandMessage);
            } catch (Exception e) {
            }
        }
    }

    public static void main(String args[]) throws Exception {
        StudentClient studentClient = new StudentClient();
        studentClient.clients();
        studentClient.run("client");
    }
}

正如布拉吉指出的，这里有很多小问题。在服务器端，主要的顺序如下：

serverSocket = new ServerSocket (8888); // socket for the server
   socket = serverSocket.accept(); // BLOCKS waiting for socket to accept client
   // ..
   printWriter = new PrintWriter(socket.getOutputStream(),true);

这意味着 printWriter 在服务器侦听、阻止等待并接受来自客户端的连接之前，它甚至不存在。
如果您想打开连接进行读写，而不想从客户端发送任何信息，请从客户端发送握手。您可以复制smtp，并使用 HELO <myname> . 甚至告诉服务器是谁在打电话。
进一步阅读后更新：
我一直都像你一样，使用隐式连接，当你使用 getOutputStream() 在客户端。然而， Socket 允许您使用socket#connect（）手动连接现有的套接字。试试看，对你来说，也许比握手更有效。