Java多线程synchronized同步

非线程安全问题

“非线程安全”问题存在于“实例变量”中，如果是方法内部的私有变量，则不存在“非线程问题”。也即是说，方法中的变量永远是线程安全的。

如果多个线程共同访问1个对象中的实例变量，则可能线程不安全。下面以实例说明

public class HasSelfNum {
    private int num = 0;
    public void add(String name) {
        try {
            if (name.equals("a")) {
                num = 100;
                System.out.println("a over");
                Thread.sleep(1000);
            } else {
                num = 200;
                System.out.println("b over");
            }
            System.out.println(name+" "+num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

public class ThreadA extends Thread{
    private HasSelfNum hasSelfNum;
    
    public ThreadA(HasSelfNum hasSelfNum) {
        this.hasSelfNum = hasSelfNum;
    }
    
    @Override
    public void run() {
        super.run();
        hasSelfNum.add("a");
    }
}

public class ThreadB extends Thread{
    private HasSelfNum hasSelfNum;
    
    public ThreadB(HasSelfNum hasSelfNum) {
        this.hasSelfNum = hasSelfNum;
    }
    
    @Override
    public void run() {
        super.run();
        hasSelfNum.add("b");
    }
}

public class Main {
    public static void main(String[] args) {
        HasSelfNum hasSelfNum = new HasSelfNum();
        ThreadA threadA = new ThreadA(hasSelfNum);
        ThreadB threadB = new ThreadB(hasSelfNum);
        
        threadA.start();
        threadB.start();
    }
}

Result

a over
b over
b 200
a 200

非线程安全！

如何处理？

在add方法上加上关键字 synchronized

synchronized public void add(String name)

Result

a over
a 100
b over
b 200

线程安全，同步访问add()方法

如果是多个对象的情况？

将上面的Main.java进行修改

public class Main {
    public static void main(String[] args) {
        HasSelfNum hasSelfNum = new HasSelfNum();
        HasSelfNum hasSelfNum2 = new HasSelfNum(); //两个对象
        
        ThreadA threadA = new ThreadA(hasSelfNum);
        ThreadB threadB = new ThreadB(hasSelfNum2);
        
        threadA.start();
        threadB.start();
    }
}

Result

a over
b over
b 200
a 100

两个线程分别访问同一个类的不同实例的相同同步方法，产生了两个锁，运行的结果是异步的。

由此可以看出，关键字synchronized取得的锁是对象锁

 

若类中既有synchronized类型方法又有非synchronized类型方法

public class MyObject {
    synchronized public void methodA(){
        try {
            System.out.println(Thread.currentThread().getName()+" begin");
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName()+" end");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    
    public void methodB(){
        try {
            System.out.println(Thread.currentThread().getName()+" begin");
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName()+" end");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
        
}

public class ThreadA extends Thread{
    MyObject myObject;
    public ThreadA(MyObject    myObject) {
        this.myObject = myObject;
    }
    
    @Override
    public void run() {
        super.run();
        myObject.methodA();
    }
}

public class ThreadB extends Thread{
    MyObject myObject;
    public ThreadB(MyObject    myObject) {
        this.myObject = myObject;
    }
    
    @Override
    public void run() {
        super.run();
        myObject.methodB();
    }
}

public class Main {
    public static void main(String[] args) {
        MyObject myObject = new MyObject();
        
        ThreadA threadA = new ThreadA(myObject);
        ThreadB threadB = new ThreadB(myObject);
        threadA.setName("A");
        threadB.setName("B");
        
        threadA.start();
        threadB.start();
    }
}

Result

A begin
B begin
B end
A end

线程A持有myObject对象锁，但线程B仍可以异步调用非synchronized类型方法

当在methodB方法前也加上synchronized关键字时

Result

A begin
A end
B begin
B end

线程A、B以同步的方式执行对象中的两方法

synchronized同步代码块

当synchronized修饰的方法里有个耗时很长的代码时，效率是很低的。同步代码块分为：

1.synchronized(this)

在方法中不在同步代码块中的代码异步执行，而在其中的代码同步执行

同个对象中的synchronized(this)使用的对象监视器是同一个，也就是说，当一个线程访问某个synchronized(this)代码块里面的方法时，其他线程访问其他synchronized(this)块里面的方法是会被阻塞的。另外synchronized(this)使用的监视器和syncheronized 修饰方法一致是当前对象，也会阻塞synchronized修饰的方法。

2.synchronized(obj) 非this

多个线程同时访问时只能阻塞synchronized(obj)的代码块。

同时如果锁是同一个myObject对象，还是会阻塞的

public class MyObject {
    synchronized public void methodA() {
        try {
            System.out.println(Thread.currentThread().getName() + " begin");
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName() + " end");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    public void methodB(MyObject myObject) {
        synchronized (myObject) {
            try {
                System.out.println(Thread.currentThread().getName() + " begin");
                Thread.sleep(1000);
                System.out.println(Thread.currentThread().getName() + " end");
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

public class ThreadA extends Thread{
    MyObject myObject;
    public ThreadA(MyObject    myObject) {
        this.myObject = myObject;
    }
    
    @Override
    public void run() {
        super.run();
        myObject.methodA();
    }
}

public class ThreadB extends Thread{
    MyObject myObject;
    public ThreadB(MyObject    myObject) {
        this.myObject = myObject;
    }
    
    @Override
    public void run() {
        super.run();
        myObject.methodB(myObject);
    }
}

public class Main {
    public static void main(String[] args) {
        MyObject myObject = new MyObject();
        
        ThreadA threadA = new ThreadA(myObject);
        ThreadB threadB = new ThreadB(myObject);
        threadA.setName("A");
        threadB.setName("B");
        
        threadB.start();
        threadA.start();
    }
}

Result

B begin
B end
A begin
A end

《Java多线程编程核心技术》学习