[英]java singleton thread safe
好的,這是我的問題。
我需要創建一個套接字程序,該程序可以處理來自客戶端應用程序的多個連接(我們稱其為apps1)。 我使用線程處理此問題(因此每個連接都被拋出一個新線程)
問題是我可以接受所有打開的連接的請求,但是當我要發送響應時,我只能通過最新的連接發送它。 因此,如果我有3個連接(con1,con2,con3),我可以接受來自con1,con2和con3的請求,但是我必須通過con3發送響應(假設con3是最新的連接)
我想到了使用帶有PrintWriter參數的單例。 因此,每當有一個新的連接時,它們都會調用單例並更新參數,並且當我想發送響應時,在發送之前我首先獲得了PrintWriter。
這是我的Singleton課程:
public class Singleton {
private static final Singleton instance = new Singleton();
PrintWriter out;
public static Singleton getInstance() {
return instance;
}
public Singleton ()
{
if (instance != null) {
throw new IllegalStateException("Already instantiated");
}
}
public PrintWriter getPrintWriter ()
{
return this.out;
}
public void updatePrintWriter (PrintWriter out){
this.out = out;
}
}
這是我的主要程序:
public class SocketAccept{
private ServerSocket mainSocket;
private Socket clientSocket;
public SocketAccept (int portNumber) {
Singleton s = Singleton.getInstance();
do {
try {
mainSocket = new ServerSocket(portNumber);
clientSocket = mainSocket.accept();
s.updatePrintWriter(new PrintWriter(clientSocket.getOutputStream(), true));
ClientThread (clientSocket);
} catch (IOException ex) {
Logger.getLogger(TestClass.class.getName()).log(Level.SEVERE, null, ex);
}
}while (true);//need to change this into thread pool or connection pool
}
}
這是我處理套接字的線程:
public class ClientThread extends Thread {
private Socket cs;
Singleton s = Singleton.getInstance();
PrintWriter out;
private String read(Socket sc) {
String request = "";
//read request here
return request;
}
private String process(String request) {
String response = "";
//process request here
return response;
}
public ClientThread(Socket clientSocket) {
this.cs = clientSocket;
}
@Override
public void run() {
String requestMsg = "";
String responseMsg = "";
do {
requestMsg = read(cs);// read the message
if (requestMsg.equalsIgnoreCase("SHUTDOWN")) {
break;
}
responseMsg = process(requestMsg);
out = s.getPrintWriter();
out.write(responseMsg);
} while (true);
}
}
我做對了嗎? 還是單身不可能做到這一點?
謝謝您的幫助。
不幸的是,這不是Singleton模式的線程安全實現。 在這種情況下,我也不認為您需要一個, AtomicReference可能也可以正常工作。 嘗試這個:
public class SocketAccept{
private ServerSocket mainSocket;
private Socket clientSocket;
private final AtomicReference<PrintWriter> printWriterHolder = new AtomicReference(null);
public SocketAccept (int portNumber) {
Singleton s = Singleton.getInstance();
do {
try {
mainSocket = new ServerSocket(portNumber);
clientSocket = mainSocket.accept();
printWriterHolder.set(new PrintWriter(clientSocket.getOutputStream(), true));
Thread clientThread = new ClientThread (clientSocket, printWriterHolder);
clientThread.start();
} catch (IOException ex) {
Logger.getLogger(TestClass.class.getName()).log(Level.SEVERE, null, ex);
}
}while (true);//need to change this into thread pool or connection pool
}
}
...
public class ClientThread extends Thread
...
private final AtomicReference<PrintWriter> printWriterHolder;
public ClientThread(Socket clientSocket, AtomicReference<PrintWriter> holder) {
this.cs = clientSocket;
this.printWriterHolder = holder;
}
@Override
public void run() {
String requestMsg = "";
String responseMsg = "";
do {
requestMsg = read(cs);// read the message
if (requestMsg.equalsIgnoreCase("SHUTDOWN")) {
break;
}
responseMsg = process(requestMsg);
out = printWriterHolder.get();
out.write(responseMsg);
} while (true);
}
}
如果您確實要使用Singleton模式,請參考SO的參考,其中該問題對於創建Singleton具有良好的線程安全實現: Java Singleton Pattern
您還需要根據需要通過使用synchronized
, Lock
或原子操作( AtomicInteger
, AtomicReference
等)來使訪問Singleton的狀態線程安全。
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