E:\bench-cluster\mybatis_in_action_eclipse\xmpp\src>javac com/communication/qq/ServerPlainTest.java
com\communication\qq\ServerPlainTest.java:14: 错误: 找不到符号new Thread(new ReceiveHandler(s, Role.SERVER)).start();^符号:   类 ReceiveHandler位置: 类 ServerPlainTest
com\communication\qq\ServerPlainTest.java:15: 错误: 找不到符号new Thread(new SendHandler(s, Role.SERVER)).start();^符号:   类 SendHandler位置: 类 ServerPlainTest
2 个错误

2）reasons of errors:  参考一下代码，ServerPlainTest 依赖于 ReceiverHandler.java  and SendHandler.java ；而 ReceiverHandler.java and SendHandler.java 定义在 MessageHandler.java ，它们属于包内可见性，包外不可见； 由编译报错的结果来看，当javac 编译器去编译ServerPlainTest.java的时候，它 不会去编译 包可见性的类，所以就不会产生 相应的 .class 文件，所以报错；

public class ServerPlainTest { // 包内包外可见public static void main(String[] args) {try {ServerSocket ss = new ServerSocket(8189);System.out.println("the server has startuped, waiting for connections.");while (true) { // accept multiple clients connection request.Socket s = ss.accept();System.out.println("a client has connected successfully.");new Thread(new ReceiveHandler(s, Role.SERVER)).start();new Thread(new SendHandler(s, Role.SERVER)).start();}} catch (Exception e) {System.out.println(e);}}
}

public class MessageHandler { // 包内包外可见.
}// receive msg.
class ReceiveHandler implements Runnable { // 包内可见性.private Socket socket;private Role role;public ReceiveHandler(Socket socket, Role role) {this.socket = socket;this.role = role;       }public void run() {try {System.out.println(role + " executes ReceiveHandler.run method.");InputStream is = socket.getInputStream();Scanner in = new Scanner(is);// echo client inputboolean done = false;while (!done && in.hasNextLine()) {String line = in.nextLine();System.out.println("from "+ role + ": " + line);if (line.trim().equalsIgnoreCase("bye")) {done = true;}}// close relative instance.in.close();} catch (IOException e) {e.printStackTrace();} }
}// send msg.
class SendHandler implements Runnable { // 包内可见性.private Socket socket;private Role role;public SendHandler(Socket socket, Role role) {this.socket = socket;this.role = role;     }public void run() {PrintWriter pw = null;try {System.out.println(role + " executes SendHandler.run method.");Scanner s = new Scanner(System.in);OutputStream os = socket.getOutputStream();pw = new PrintWriter(os, true); // autoFlush=true.if (role.equals(Role.SERVER)) {pw.println("from server: you've connected with the server.");}while (s.hasNext()) {String line = s.nextLine();pw.println(line);}pw.println("connection closed successfully. bye!");} catch (Exception e) {e.printStackTrace();}  }
}

【2】solution：在编译 ServerPlainTest.java 之前，编译 MessageHandler.java， 这样会附带编译 ReceiverHandler.java and SendHandler.java ；

E:\bench-cluster\mybatis_in_action_eclipse\xmpp\src>javac com/communication/qq/ServerPlainTest.java com/communication/qq/MessageHandler.java