java 个未通过双重锁定检查的代码

Question

注意：我知道在java5（2004年给出）之前，double checking lock会在java失败，即使你添加"volatile" 到 "instance" 字段。在 java 5 之后，volatile 语义适用于双重检查锁。我也知道 without "volatile", double checking lock will fail 即使在 java5 之后，由于乱序执行。但是，我的问题是：如何编写代码来证明（没有 "volatile" 双重检查锁会失败）？？？

我读过很多文章说双重检查锁会在 java 中失败，所以我认为下面的代码试图获取 SocketFactory 的单例实例，将会失败（因为 "instance"字段不可变）：

  private static SocketFactory instance  = null;
  private static SocketFactory getInstance() {
      if (instance == null) {
         synchronized (SocketFactory.class){
            if (instance == null){
               instance = new SocketFactory(); // line 1
            }
         }
      }
      return instance;
   }

但是，问题是，如果上面的代码会失败，我该如何证明呢？我试着写下面的代码（我想，如果第 1 行将被重新排序，"instance" 引用可能指向一个 "constructed" 字段为 "false" 的 SocketFactory 对象：

import java.io.*;
import java.nio.*;
import java.nio.file.*;

public class TIJ_doubleCheckingLock {

    static TIJ_doubleCheckingLock instance;

    private Object lock;

    private boolean constructed = false;

    {   
        constructed = false;
    }

    private TIJ_doubleCheckingLock() {
        try{
            Thread.sleep(10000);
        } catch(Exception e) {
        }
        constructed = true;
    }

    static TIJ_doubleCheckingLock getInstance() {
        if (instance == null) {
            synchronized (TIJ_doubleCheckingLock.class) {
                try{
                    Thread.sleep(1000);
                } catch(Exception e) {

                }
                if(instance == null) {
                    instance = new TIJ_doubleCheckingLock();
                }
            }
        }
        return instance;
    }

    public static void main(String args[]) {
        class MyThread extends Thread {
                @Override
                public void run() {

                    TIJ_doubleCheckingLock instance = TIJ_doubleCheckingLock.getInstance();
                    String fileName = "TIJ_doubleCheckingLockResult.txt";
                    java.io.File file = new File(fileName);
                    try {
                        if(!instance.constructed) {
                            java.nio.file.Files.write(Paths.get(file.toURI()), (instance.constructed+"").
                                    getBytes("utf-8"), StandardOpenOption.CREATE, StandardOpenOption.APPEND);
                        }
                    } catch (Exception e) {
                    }
                }

        }

        Thread firstThread = new MyThread();
        firstThread.start();

        try{
            Thread.sleep(5000);
        } catch(Exception e) {}


        for(int i=0;i<10;i++) {
            Thread thread = new MyThread(); 
            thread.start();
        }
    }
}

但我从未在 txt 文件中看到 "false"。那么如何证明双重检查锁会失败呢？

Answer 1

双重检查锁定was broken in Java. Your code works fine with Java 5 and onwards. The JVM since got a new memory model。

您的代码还需要 volatile 关键字。

private static volatile SocketFactory instance;

Answer 2

你需要知道我的代码在synchronized中的区别：

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/**
 * @author heyunxia (love3400wind@163.com)
 * @version 1.0
 * @since 2016-01-15 下午3:50
 */
public class SocketFactoryTest {

    public static void main(String[] args) {
        SocketFactoryTest test = new SocketFactoryTest();
        test.run();

    }

    private static final int COUNTER = 20;

    CyclicBarrier barrier = new CyclicBarrier(COUNTER);

    public void run(){
        for(int i=0; i<COUNTER; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    SocketFactory.getInstance(barrier);
                }
            }).start();
        }
    }

}


class SocketFactory {
    private static SocketFactory instance = null;

    public static SocketFactory getInstance(CyclicBarrier barrier) {
        if (instance == null) {
            try {
                barrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
            synchronized (SocketFactory.class) {
                // option 1
                instance = new SocketFactory();
                System.out.println(Thread.currentThread().getName() + "***");

                /*
                // option 2
                if (instance == null) {
                    System.out.println(Thread.currentThread().getName() + "***");
                    instance = new SocketFactory();
                }else {
                    System.out.println(Thread.currentThread().getName() + "-have already instanced...");
                }*/
            }
        }
        return instance;
    }
}

java 个未通过双重锁定检查的代码

java codes that fails double locking checking

java

concurrency

singleton

double-checked-locking