Java多執行緒（三）：Synchronized

Rest探路者發表於2019-07-02

多執行緒安全

髒讀：多個執行緒對同一個物件的例項變數進行修改後訪問，導致讀到的資料是被修改過的。

例項

ThreadDomain16類

public class ThreadDomain16 {
    private int num = 0;

    public void addNum(String userName)
    {
        try
        {
            if ("a".equals(userName))
            {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            }
            else
            {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(userName + " num = " + num);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread16_0類

public class MyThread16_0 extends Thread{
    private ThreadDomain16 td;

    public MyThread16_0(ThreadDomain16 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.addNum("a");
    }
}

MyThread16_1類
啟動MyThread16_0 和MyThread16_1 執行緒

public class MyThread16_1 extends Thread {
    private ThreadDomain16 td;

    public MyThread16_1(ThreadDomain16 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.addNum("b");
    }

}

main方法

public class MyThread16_main_0 {
    public static void main(String[] args)
    {
        ThreadDomain16 td = new ThreadDomain16();
        MyThread16_0 mt0 = new MyThread16_0(td);
        MyThread16_1 mt1 = new MyThread16_1(td);
        mt0.start();
        mt1.start();
    }
}

輸出結果如下

a set over!
b set over!
b num = 200
a num = 200

理論上應該輸出a num = 100,b num = 200，結果a num = 200。
我們來分析下執行流程，MyThread16_0執行緒修改num為100，此時sleep兩秒，在MyThread16_0 sleep的過程中，MyThread16_1執行緒修改num為200。導致MyThread16_0的num也變成200。最後先輸出b num = 200，再輸出a num = 200。出現了髒讀。
解決該問題很簡單，我們在addNum()前面加上synchronized關鍵字

public synchronized void addNum(String userName)
    {
        try
        {
            if ("a".equals(userName))
            {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            }
            else
            {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(userName + " num = " + num);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

輸出結果如下

a set over!
a num = 100
b set over!
b num = 200

按序列印，synchronized修飾的方法形成了鎖，哪個執行緒先執行了synchronized修飾的方法哪個執行緒就先獲得了鎖，其他執行緒必須等待，直到該執行緒釋放鎖，等待的執行緒獲得鎖方可執行。

一個物件一個鎖

同步情況下，使用如下main方法

public class MyThread16_main_1 {
    public static void main(String[] args)
    {
        ThreadDomain16 td0 = new ThreadDomain16();
        ThreadDomain16 td1 = new ThreadDomain16();
        MyThread16_0 mt0 = new MyThread16_0(td0);
        MyThread16_1 mt1 = new MyThread16_1(td1);
        mt0.start();
        mt1.start();
    }
}

執行結果

a set over!
b set over!
b num = 200
a num = 100

出現了交叉列印，b沒有等待a執行結束，說明synchronized是物件鎖，而不是方法鎖。這裡宣告瞭兩個ThreadDomain16物件，所以有兩個物件鎖，不受“等待鎖的釋放”的制約。
那麼我們把addNum方法前面的synchronized關鍵字去掉，非同步情況下
執行結果如下

a set over!
b set over!
b num = 200
a num = 100

可以看到，沒有synchronized的幫助，依然避免了髒讀現象。例項變數num為每個物件獨自佔有。但是，這種解決方式需要宣告多個物件，不推薦。

synchronized方法與鎖物件

現在看一個例子，一個方法有synchronized修飾，另一個方法無synchronized修飾
ThreadDomain17類

public class ThreadDomain17 {
    public synchronized void methodA()
    {
        try
        {
            System.out.println("開始 methodA, 執行緒名稱 = " +
                    Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("結束 methodA, 執行緒名稱 = " +
                    Thread.currentThread().getName());
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    public void methodB()
    {
        try
        {
            System.out.println("開始 methodB, 執行緒名稱 = " +
                    Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("結束 methodB, 執行緒名稱 = " +
                    Thread.currentThread().getName());
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread17_0類

public class MyThread17_0 extends Thread{
    private ThreadDomain17 td;

    public MyThread17_0(ThreadDomain17 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.methodA();
    }
}

MyThread17_1類

public class MyThread17_1 extends Thread{
    private ThreadDomain17 td;

    public MyThread17_1(ThreadDomain17 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.methodB();
    }
}

main方法

public class MyThread17_main {
    public static void main(String[] args) {
        ThreadDomain17 td = new ThreadDomain17();
        MyThread17_0 mt0 = new MyThread17_0(td);
        mt0.setName("A");
        MyThread17_1 mt1 = new MyThread17_1(td);
        mt1.setName("B");
        mt0.start();
        mt1.start();
    }
}

執行結果如下

開始 methodB, 執行緒名稱 = B
開始 methodA, 執行緒名稱 = A
結束 methodA, 執行緒名稱 = A
結束 methodB, 執行緒名稱 = B

可以看到，非synchronized的methodB方法以非同步方式被呼叫。
把MyThread17_1類的run方法改成td.methodA();
輸出結果如下

開始 methodA, 執行緒名稱 = A
結束 methodA, 執行緒名稱 = A
開始 methodA, 執行緒名稱 = B
結束 methodA, 執行緒名稱 = B

待執行的執行緒在等待synchronized方法同步再執行。
A執行緒持有ThreadDomain17物件鎖，B執行緒可以非同步方式呼叫ThreadDomain17物件的非synchronized方法；
A執行緒持有ThreadDomain17物件鎖，B執行緒呼叫ThreadDomain17物件的synchronized方法需要等待；
把MyThread17_1類的run方法改回td.methodB();把ThreadDomain17類的methodB方法加上synchronized關鍵字，輸出結果如下

開始 methodA, 執行緒名稱 = A
結束 methodA, 執行緒名稱 = A
開始 methodB, 執行緒名稱 = B
結束 methodB, 執行緒名稱 = B

這個很好理解，先執行的執行緒獲得同步鎖，等該執行緒必須執行完畢，等待的執行緒才能執行，所有的操作都是同步的。

synchronized鎖重入

synchronized是可重入鎖，當一個執行緒得到物件鎖之後，可以再次申請物件鎖並獲得物件鎖。
ThreadDomain18類

public class ThreadDomain18 {
    public synchronized void print1()
    {
        System.out.println("ThreadDomain18.print1()");
        print2();
    }

    public synchronized void print2()
    {
        System.out.println("ThreadDomain18.print2()");
        print3();
    }

    public synchronized void print3()
    {
        System.out.println("ThreadDomain18.print3()");
    }
}

MyThread18類

public class MyThread18 extends Thread{
    public void run()
    {
        ThreadDomain18 td = new ThreadDomain18();
        td.print1();
    }

    public static void main(String[] args)
    {
        MyThread18 mt = new MyThread18();
        mt.start();
    }
    
}

結果如下

ThreadDomain18.print1()
ThreadDomain18.print2()
ThreadDomain18.print3()

結果說明，MyThread18在有鎖的情況下多次獲取到物件鎖

異常釋放鎖

ThreadDomain19類

public class ThreadDomain19 {
    public synchronized void testMethod()
    {
        try
        {
            System.out.println("進入 ThreadDomain19.testMethod(), currentThread = " +
                    Thread.currentThread().getName());
            long l = 5;
            while (true)
            {
                long lo = 2 / l;
                Thread.sleep(1000);
                l--;
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

MyThread19類

public class MyThread19 extends Thread {
    private ThreadDomain19 td;

    public MyThread19(ThreadDomain19 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.testMethod();
    }

    public static void main(String[] args)
    {
        ThreadDomain19 td = new ThreadDomain19();
        MyThread19 mt0 = new MyThread19(td);
        MyThread19 mt1 = new MyThread19(td);
        mt0.start();
        mt1.start();
    }
}

輸出結果如下

進入 ThreadDomain19.testMethod(), currentThread = Thread-1
java.lang.ArithmeticException: / by zero
進入 ThreadDomain19.testMethod(), currentThread = Thread-0
    at com.advance.MultiThread3.ThreadDomain19.testMethod(ThreadDomain19.java:18)
    at com.advance.MultiThread3.MyThread19.run(MyThread19.java:18)
java.lang.ArithmeticException: / by zero
    at com.advance.MultiThread3.ThreadDomain19.testMethod(ThreadDomain19.java:18)
    at com.advance.MultiThread3.MyThread19.run(MyThread19.java:18)

由上可見，一旦出現異常，鎖自動釋放。Thread-1剛拋完異常，此時Thread-1釋放了鎖，Thread-0就開始執行。

synchronized方法塊

ThreadDomain20類

public class ThreadDomain20 extends Thread{
    public void doLongTimeTask() throws Exception
    {
        for (int i = 0; i < 100; i++)
        {
            System.out.println("nosynchronized threadName = " +
                    Thread.currentThread().getName() + ", i = " + (i + 1));
        }
        System.out.println();
        synchronized (this){
            for (int i = 0; i < 100; i++)
            {
                System.out.println("synchronized threadName = " +
                        Thread.currentThread().getName() + ", i = " + (i + 1));
            }
        }
    }
    
}

MyThread20類

public class MyThread20 extends Thread{
    private ThreadDomain20 td;

    public MyThread20(ThreadDomain20 td)
    {
        this.td = td;
    }

    public void run()
    {
        try
        {
            td.doLongTimeTask();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    public static void main(String[] args)
    {
        ThreadDomain20 td = new ThreadDomain20();
        MyThread20 mt0 = new MyThread20(td);
        mt0.setName("A");
        MyThread20 mt1 = new MyThread20(td);
        mt1.setName("B");
        mt0.start();
        mt1.start();
    }

}

執行結果如下

nosynchronized threadName = A, i = 1
nosynchronized threadName = B, i = 1
nosynchronized threadName = A, i = 2
nosynchronized threadName = B, i = 2
nosynchronized threadName = A, i = 3
nosynchronized threadName = B, i = 3
nosynchronized threadName = A, i = 4
nosynchronized threadName = B, i = 4
nosynchronized threadName = A, i = 5
nosynchronized threadName = B, i = 5
nosynchronized threadName = A, i = 6
nosynchronized threadName = B, i = 6
nosynchronized threadName = A, i = 7
nosynchronized threadName = B, i = 7
nosynchronized threadName = A, i = 8
nosynchronized threadName = B, i = 8
nosynchronized threadName = A, i = 9
nosynchronized threadName = B, i = 9
nosynchronized threadName = A, i = 10
nosynchronized threadName = B, i = 10
nosynchronized threadName = A, i = 11
nosynchronized threadName = B, i = 11
nosynchronized threadName = A, i = 12
nosynchronized threadName = B, i = 12
nosynchronized threadName = A, i = 13
nosynchronized threadName = B, i = 13
nosynchronized threadName = A, i = 14
nosynchronized threadName = B, i = 14
nosynchronized threadName = A, i = 15
nosynchronized threadName = A, i = 16
nosynchronized threadName = A, i = 17
nosynchronized threadName = A, i = 18
nosynchronized threadName = B, i = 15
nosynchronized threadName = A, i = 19
nosynchronized threadName = B, i = 16
nosynchronized threadName = A, i = 20
nosynchronized threadName = B, i = 17
nosynchronized threadName = A, i = 21
nosynchronized threadName = B, i = 18
nosynchronized threadName = A, i = 22
nosynchronized threadName = B, i = 19
nosynchronized threadName = A, i = 23
nosynchronized threadName = A, i = 24
nosynchronized threadName = B, i = 20
nosynchronized threadName = A, i = 25
nosynchronized threadName = B, i = 21
nosynchronized threadName = A, i = 26
nosynchronized threadName = B, i = 22
nosynchronized threadName = A, i = 27
nosynchronized threadName = B, i = 23
nosynchronized threadName = A, i = 28
nosynchronized threadName = B, i = 24
nosynchronized threadName = A, i = 29
nosynchronized threadName = B, i = 25
nosynchronized threadName = A, i = 30
nosynchronized threadName = B, i = 26
nosynchronized threadName = A, i = 31
nosynchronized threadName = A, i = 32
nosynchronized threadName = B, i = 27
nosynchronized threadName = A, i = 33
nosynchronized threadName = B, i = 28
nosynchronized threadName = A, i = 34
nosynchronized threadName = B, i = 29
nosynchronized threadName = A, i = 35
nosynchronized threadName = B, i = 30
nosynchronized threadName = A, i = 36
nosynchronized threadName = B, i = 31
nosynchronized threadName = A, i = 37
nosynchronized threadName = B, i = 32
nosynchronized threadName = A, i = 38
nosynchronized threadName = A, i = 39
nosynchronized threadName = B, i = 33
nosynchronized threadName = A, i = 40
nosynchronized threadName = B, i = 34
nosynchronized threadName = A, i = 41
nosynchronized threadName = B, i = 35
nosynchronized threadName = A, i = 42
nosynchronized threadName = B, i = 36
nosynchronized threadName = A, i = 43
nosynchronized threadName = B, i = 37
nosynchronized threadName = A, i = 44
nosynchronized threadName = B, i = 38
nosynchronized threadName = A, i = 45
nosynchronized threadName = B, i = 39
nosynchronized threadName = A, i = 46
nosynchronized threadName = B, i = 40
nosynchronized threadName = A, i = 47
nosynchronized threadName = B, i = 41
nosynchronized threadName = A, i = 48
nosynchronized threadName = B, i = 42
nosynchronized threadName = A, i = 49
nosynchronized threadName = B, i = 43
nosynchronized threadName = A, i = 50
nosynchronized threadName = B, i = 44

nosynchronized threadName = B, i = 45
synchronized threadName = A, i = 1
nosynchronized threadName = B, i = 46
synchronized threadName = A, i = 2
nosynchronized threadName = B, i = 47
synchronized threadName = A, i = 3
nosynchronized threadName = B, i = 48
synchronized threadName = A, i = 4
nosynchronized threadName = B, i = 49
nosynchronized threadName = B, i = 50

synchronized threadName = A, i = 5
synchronized threadName = A, i = 6
synchronized threadName = A, i = 7
synchronized threadName = A, i = 8
synchronized threadName = A, i = 9
synchronized threadName = A, i = 10
synchronized threadName = A, i = 11
synchronized threadName = A, i = 12
synchronized threadName = A, i = 13
synchronized threadName = A, i = 14
synchronized threadName = A, i = 15
synchronized threadName = A, i = 16
synchronized threadName = A, i = 17
synchronized threadName = A, i = 18
synchronized threadName = A, i = 19
synchronized threadName = A, i = 20
synchronized threadName = A, i = 21
synchronized threadName = A, i = 22
synchronized threadName = A, i = 23
synchronized threadName = A, i = 24
synchronized threadName = A, i = 25
synchronized threadName = A, i = 26
synchronized threadName = A, i = 27
synchronized threadName = A, i = 28
synchronized threadName = A, i = 29
synchronized threadName = A, i = 30
synchronized threadName = A, i = 31
synchronized threadName = A, i = 32
synchronized threadName = A, i = 33
synchronized threadName = A, i = 34
synchronized threadName = A, i = 35
synchronized threadName = A, i = 36
synchronized threadName = A, i = 37
synchronized threadName = A, i = 38
synchronized threadName = A, i = 39
synchronized threadName = A, i = 40
synchronized threadName = A, i = 41
synchronized threadName = A, i = 42
synchronized threadName = A, i = 43
synchronized threadName = A, i = 44
synchronized threadName = A, i = 45
synchronized threadName = A, i = 46
synchronized threadName = A, i = 47
synchronized threadName = A, i = 48
synchronized threadName = A, i = 49
synchronized threadName = A, i = 50
synchronized threadName = B, i = 1
synchronized threadName = B, i = 2
synchronized threadName = B, i = 3
synchronized threadName = B, i = 4
synchronized threadName = B, i = 5
synchronized threadName = B, i = 6
synchronized threadName = B, i = 7
synchronized threadName = B, i = 8
synchronized threadName = B, i = 9
synchronized threadName = B, i = 10
synchronized threadName = B, i = 11
synchronized threadName = B, i = 12
synchronized threadName = B, i = 13
synchronized threadName = B, i = 14
synchronized threadName = B, i = 15
synchronized threadName = B, i = 16
synchronized threadName = B, i = 17
synchronized threadName = B, i = 18
synchronized threadName = B, i = 19
synchronized threadName = B, i = 20
synchronized threadName = B, i = 21
synchronized threadName = B, i = 22
synchronized threadName = B, i = 23
synchronized threadName = B, i = 24
synchronized threadName = B, i = 25
synchronized threadName = B, i = 26
synchronized threadName = B, i = 27
synchronized threadName = B, i = 28
synchronized threadName = B, i = 29
synchronized threadName = B, i = 30
synchronized threadName = B, i = 31
synchronized threadName = B, i = 32
synchronized threadName = B, i = 33
synchronized threadName = B, i = 34
synchronized threadName = B, i = 35
synchronized threadName = B, i = 36
synchronized threadName = B, i = 37
synchronized threadName = B, i = 38
synchronized threadName = B, i = 39
synchronized threadName = B, i = 40
synchronized threadName = B, i = 41
synchronized threadName = B, i = 42
synchronized threadName = B, i = 43
synchronized threadName = B, i = 44
synchronized threadName = B, i = 45
synchronized threadName = B, i = 46
synchronized threadName = B, i = 47
synchronized threadName = B, i = 48
synchronized threadName = B, i = 49
synchronized threadName = B, i = 50

結論：
A執行緒訪問synchronized方法塊的時候，B執行緒依然可以訪問非synchronized方法塊的內容；
A執行緒訪問synchronized方法塊的時候，B執行緒訪問synchronized方法塊會被阻塞；

synchronized方法塊和synchronized方法

synchronized關鍵字修飾的方法和synchronized方法塊修飾的方法效果一致。synchronized方法塊可以提高程式碼執行效率，在需要同步的地方去同步。
ThreadDomain21類

public class ThreadDomain21 {
    public synchronized void otherMethod()
    {
        System.out.println("----------run--otherMethod");
    }

    public void doLongTask()
    {
        synchronized (this)
        {
            for (int i = 0; i < 100; i++)
            {
                System.out.println("synchronized threadName = " +
                        Thread.currentThread().getName() + ", i = " + (i + 1));
                try
                {
                    Thread.sleep(5);
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
        }
    }
}

MyThread21_0類

public class MyThread21_0 extends Thread{
    private ThreadDomain21 td;

    public MyThread21_0(ThreadDomain21 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.doLongTask();
    }
}

MyThread21_1類

public class MyThread21_1 extends Thread{
    private ThreadDomain21 td;

    public MyThread21_1(ThreadDomain21 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.otherMethod();
    }
}

main方法，sleep一會，讓mt0先執行。

public class MyThread21_main {
    public static void main(String[] args) throws Exception
    {
        ThreadDomain21 td = new ThreadDomain21();
        MyThread21_0 mt0 = new MyThread21_0(td);
        mt0.setName("A");
        MyThread21_1 mt1 = new MyThread21_1(td);
        mt1.setName("B");
        mt0.start();
        Thread.sleep(100);
        mt1.start();
    }
}

輸出結果如下

synchronized threadName = A, i = 1
synchronized threadName = A, i = 2
synchronized threadName = A, i = 3
synchronized threadName = A, i = 4
synchronized threadName = A, i = 5
synchronized threadName = A, i = 6
synchronized threadName = A, i = 7
synchronized threadName = A, i = 8
synchronized threadName = A, i = 9
synchronized threadName = A, i = 10
synchronized threadName = A, i = 11
synchronized threadName = A, i = 12
synchronized threadName = A, i = 13
synchronized threadName = A, i = 14
synchronized threadName = A, i = 15
synchronized threadName = A, i = 16
synchronized threadName = A, i = 17
synchronized threadName = A, i = 18
synchronized threadName = A, i = 19
synchronized threadName = A, i = 20
synchronized threadName = A, i = 21
synchronized threadName = A, i = 22
synchronized threadName = A, i = 23
......
synchronized threadName = A, i = 89
synchronized threadName = A, i = 90
synchronized threadName = A, i = 91
synchronized threadName = A, i = 92
synchronized threadName = A, i = 93
synchronized threadName = A, i = 94
synchronized threadName = A, i = 95
synchronized threadName = A, i = 96
synchronized threadName = A, i = 97
synchronized threadName = A, i = 98
synchronized threadName = A, i = 99
synchronized threadName = A, i = 100
----------run--otherMethod

可以看到，otherMethod方法被doLongTask方法阻塞，說明synchronized鎖定了整個物件，把otherMethod方法synchronized關鍵字去掉，輸出結果如下

synchronized threadName = A, i = 1
synchronized threadName = A, i = 2
synchronized threadName = A, i = 3
synchronized threadName = A, i = 4
synchronized threadName = A, i = 5
synchronized threadName = A, i = 6
synchronized threadName = A, i = 7
synchronized threadName = A, i = 8
synchronized threadName = A, i = 9
synchronized threadName = A, i = 10
synchronized threadName = A, i = 11
synchronized threadName = A, i = 12
synchronized threadName = A, i = 13
synchronized threadName = A, i = 14
synchronized threadName = A, i = 15
synchronized threadName = A, i = 16
synchronized threadName = A, i = 17
synchronized threadName = A, i = 18
synchronized threadName = A, i = 19
synchronized threadName = A, i = 20
synchronized threadName = A, i = 21
----------run--otherMethod
......
synchronized threadName = A, i = 72
synchronized threadName = A, i = 73
synchronized threadName = A, i = 74
synchronized threadName = A, i = 75
synchronized threadName = A, i = 76
synchronized threadName = A, i = 77
synchronized threadName = A, i = 78
synchronized threadName = A, i = 79
synchronized threadName = A, i = 80
synchronized threadName = A, i = 81
synchronized threadName = A, i = 82
synchronized threadName = A, i = 83
synchronized threadName = A, i = 84
synchronized threadName = A, i = 85
synchronized threadName = A, i = 86
synchronized threadName = A, i = 87
synchronized threadName = A, i = 88
synchronized threadName = A, i = 89
synchronized threadName = A, i = 90
synchronized threadName = A, i = 91
synchronized threadName = A, i = 92
synchronized threadName = A, i = 93
synchronized threadName = A, i = 94
synchronized threadName = A, i = 95
synchronized threadName = A, i = 96
synchronized threadName = A, i = 97
synchronized threadName = A, i = 98
synchronized threadName = A, i = 99
synchronized threadName = A, i = 100

otherMethod方法非同步執行了。
結論如下：
不同的方法被synchronized修飾會產生互斥，synchronized鎖住的是物件；
synchronized方法塊和synchronized方法效果一致；

synchronized(非this物件)

ThreadDomain22類

public class ThreadDomain22 {
    private String userNameParam;
    private String passwordParam;
    private String anyString = new String();

    public void setUserNamePassword(String userName, String password)
    {
        try
        {
            synchronized (anyString)
            {
                System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                        "在 " + new Date() + " 進入同步程式碼塊");
                userNameParam = userName;
                Thread.sleep(3000);
                passwordParam = password;
                System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                        "在 " + new Date() + " 離開同步程式碼塊");
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyThread22_0類

public class MyThread22_0 extends Thread{
    private ThreadDomain22 td;

    public MyThread22_0(ThreadDomain22 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.setUserNamePassword("A", "AA");
    }
}

MyThread22_1類

public class MyThread22_1 extends Thread{
    private ThreadDomain22 td;

    public MyThread22_1(ThreadDomain22 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.setUserNamePassword("B", "B");
    }
}

main

public class MyThread22_main{
    public static void main(String[] args)
    {
        ThreadDomain22 td = new ThreadDomain22();
        MyThread22_0 mt0 = new MyThread22_0(td);
        MyThread22_1 mt1 = new MyThread22_1(td);
        mt0.start();
        mt1.start();
    }
}

輸出結果

執行緒名稱為：Thread-0在 Tue Jul 02 14:51:16 CST 2019 進入同步程式碼塊
執行緒名稱為：Thread-0在 Tue Jul 02 14:51:19 CST 2019 離開同步程式碼塊
執行緒名稱為：Thread-1在 Tue Jul 02 14:51:19 CST 2019 進入同步程式碼塊
執行緒名稱為：Thread-1在 Tue Jul 02 14:51:22 CST 2019 離開同步程式碼塊

synchronized(anyString)和synchronized(this)效果一致，anyString作為全域性變數是一個物件，如果放入try裡面，就變成了多個物件，變成了非同步執行。

再看一個例子
ThreadDomain23類

public class ThreadDomain23 {
    public void testMethod1(MyObject mo)
    {
        try
        {
            synchronized (mo)
            {
                System.out.println("testMethod1__getLock time = " +
                        new Date() + ", run ThreadName = " +
                        Thread.currentThread().getName());
                Thread.sleep(5000);
                System.out.println("testMethod1__releaseLock time = " +
                        new Date() + ", run ThreadName = " +
                        Thread.currentThread().getName());
            }
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

MyObject類

public class MyObject {
    public synchronized void speedPrintString()
    {
        System.out.println("speedPrintString__getLock time = " +
                new Date() + ", run ThreadName = " +
                Thread.currentThread().getName());
        System.out.println("speedPrintString__releaseLock time = " +
                new Date() + ", run ThreadName = " +
                Thread.currentThread().getName());
    }
}

MyThread23_0類

public class MyThread23_0 extends Thread{
    private ThreadDomain23 td;
    private MyObject mo;

    public MyThread23_0(ThreadDomain23 td, MyObject mo)
    {
        this.td = td;
        this.mo = mo;
    }

    public void run()
    {
        td.testMethod1(mo);
    }
}

MyThread23_1類

public class MyThread23_1 extends Thread {
    private MyObject mo;

    public MyThread23_1(MyObject mo)
    {
        this.mo = mo;
    }

    public void run()
    {
        mo.speedPrintString();
    }
}

main方法

public class MyThread23_main {
    public static void main(String[] args)
    {
        ThreadDomain23 td = new ThreadDomain23();
        MyObject mo = new MyObject();
        MyThread23_0 mt0 = new MyThread23_0(td, mo);
        MyThread23_1 mt1 = new MyThread23_1(mo);
        mt0.start();
        mt1.start();
    }
}

輸出結果如下

testMethod1__getLock time = Tue Jul 02 15:23:05 CST 2019, run ThreadName = Thread-0
testMethod1__releaseLock time = Tue Jul 02 15:23:10 CST 2019, run ThreadName = Thread-0
speedPrintString__getLock time = Tue Jul 02 15:23:10 CST 2019, run ThreadName = Thread-1
speedPrintString__releaseLock time = Tue Jul 02 15:23:10 CST 2019, run ThreadName = Thread-1

speedPrintString方法總是在testMethod1方法之後執行，說明ThreadDomain23獲得了鎖，mo物件中的syncronized方法被阻塞，執行完testMethod1方法才會執行speedPrintString方法。
即當一個執行緒呼叫該方法時，其他執行緒阻塞。
結論：
1.當多個執行緒同時執行synchronized(非this物件){}同步程式碼塊時呈同步效果
2.當其他執行緒執行非this物件中的synchronized同步方法時呈同步效果

synchronized靜態方法

synchronized static可以對當前的類加鎖，即類鎖。
ThreadDomain24類

public class ThreadDomain24 {
    public synchronized static void printA()
    {
        try
        {
            System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                    "在" + System.currentTimeMillis() + "進入printA()方法");
            Thread.sleep(3000);
            System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                    "在" + System.currentTimeMillis() + "離開printA()方法");
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    public synchronized static void printB()
    {
        System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "進入printB()方法");
        System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "離開printB()方法");

    }

    public synchronized void printC()
    {
        System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "進入printC()方法");
        System.out.println("執行緒名稱為：" + Thread.currentThread().getName() +
                "在" + System.currentTimeMillis() + "離開printC()方法");
    }
}

MyThread24_0類，靜態方法

public class MyThread24_0 extends Thread{
    public void run()
    {
        ThreadDomain24.printA();
    }
}

MyThread24_1類，靜態方法

public class MyThread24_1 extends Thread {
    public void run()
    {
        ThreadDomain24.printB();
    }
}

MyThread24_2類，例項化呼叫方法

public class MyThread24_2 extends Thread{
    private ThreadDomain24 td;

    public MyThread24_2(ThreadDomain24 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.printC();
    }
}

main方法

public class MyThread24_main {
    public static void main(String[] args)
    {
        ThreadDomain24 td = new ThreadDomain24();
        MyThread24_0 mt0 = new MyThread24_0();
        MyThread24_1 mt1 = new MyThread24_1();
        MyThread24_2 mt2 = new MyThread24_2(td);
        mt0.start();
        mt1.start();
        mt2.start();
    }
}

輸出結果如下

執行緒名稱為：Thread-0在1562057924755進入printA()方法
執行緒名稱為：Thread-2在1562057924755進入printC()方法
執行緒名稱為：Thread-2在1562057924755離開printC()方法
執行緒名稱為：Thread-0在1562057927755離開printA()方法
執行緒名稱為：Thread-1在1562057927755進入printB()方法
執行緒名稱為：Thread-1在1562057927755離開printB()方法

可以看到，printA方法（類鎖）和printB方法（類鎖）同步執行，printC方法（物件鎖）非同步執行，printA方法和printB方法互斥，printC不受類鎖影響。
結論：
靜態方法持有類鎖，非靜態方法持有物件鎖，兩者互不干擾。

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Java多執行緒（三）：Synchronized

多執行緒安全

例項

一個物件一個鎖

synchronized方法與鎖物件

synchronized鎖重入

異常釋放鎖

synchronized方法塊

synchronized方法塊和synchronized方法

synchronized(非this物件)

synchronized靜態方法

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