【Java基础】多线程之synchronized

2024-08-26 22:38
文章标签 java 基础 多线程 synchronized

本文主要是介绍【Java基础】多线程之synchronized，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

为什么要用synchronized
          多线程可以提高效率，但是在共享变量的情况下，可能出现混乱的情况，比如A线程将共享变量 m=5，但是B线程在用m变量的时候，出现了混乱。这样多个线程共同访问同样的一个资源，你争我夺，一团乱。线程的不安全现象一触即发。所以必须要对这种资源冲突进行预防。

      所以，在共同访问的资源进行加锁即可，第一种方法就是同步锁，也即用关键字synchronized。该关键字可以加在方法和语句块中，也即是同步方法和同步语句块。

synchronized原理

           各个线程在调用关键字synchronized方法或者语句块的时候，需要排队进行。当A线程在调用该方法（或者语句块）的时候，要在该方法（或者语句块）上锁，B线程则等待，当然可以以异步的方法调用非synchronized方法。当A线程释放了synchronized方法（或者语句块），B线程才可以调用该方法。
    
       synchronized关键字还可以实现可重入锁，也就是一个线程连续几次获取同一把锁。

案例分析
      首先是方法类，代码如下： 

package cn.itcast.java.base.synchronize;public class Task
{public void doLongTimeTask(){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));}}}
}

      接下来是自定义的线程类，这里一共有2个线程： 

package cn.itcast.java.base.synchronize;public class MyThread1 extends Thread
{private Task task;public MyThread1(Task task){super();this.task=task;}public void run(){super.run();task.doLongTimeTask();}}


package cn.itcast.java.base.synchronize;public class MyThread2 extends Thread
{private Task task;public MyThread2(Task task){super();this.task=task;}public void run(){super.run();task.doLongTimeTask();}}
    接下来是run方法：

     
package cn.itcast.java.base.synchronize;public class Run
{public static void main(String[] args)
{Task task=new Task();MyThread1 thread1=new MyThread1(task);thread1.start();MyThread2 thread2=new MyThread2(task);thread2.start();
}
}
   
     我们看一下运行结果：

     
nosynchronized threadName=Thread-0i=1
nosynchronized threadName=Thread-1i=1
nosynchronized threadName=Thread-0i=2
nosynchronized threadName=Thread-1i=2
nosynchronized threadName=Thread-0i=3
nosynchronized threadName=Thread-1i=3
nosynchronized threadName=Thread-0i=4
nosynchronized threadName=Thread-1i=4
nosynchronized threadName=Thread-0i=5
nosynchronized threadName=Thread-1i=5
nosynchronized threadName=Thread-0i=6
nosynchronized threadName=Thread-1i=6
nosynchronized threadName=Thread-0i=7
nosynchronized threadName=Thread-0i=8
nosynchronized threadName=Thread-0i=9
nosynchronized threadName=Thread-1i=7
nosynchronized threadName=Thread-0i=10
nosynchronized threadName=Thread-1i=8
nosynchronized threadName=Thread-0i=11
nosynchronized threadName=Thread-0i=12
nosynchronized threadName=Thread-0i=13
nosynchronized threadName=Thread-1i=9
nosynchronized threadName=Thread-0i=14
nosynchronized threadName=Thread-1i=10
nosynchronized threadName=Thread-0i=15
nosynchronized threadName=Thread-1i=11
nosynchronized threadName=Thread-0i=16
nosynchronized threadName=Thread-0i=17
nosynchronized threadName=Thread-0i=18
nosynchronized threadName=Thread-0i=19
nosynchronized threadName=Thread-0i=20
nosynchronized threadName=Thread-0i=21
nosynchronized threadName=Thread-0i=22
nosynchronized threadName=Thread-0i=23
nosynchronized threadName=Thread-0i=24
nosynchronized threadName=Thread-0i=25
nosynchronized threadName=Thread-0i=26
nosynchronized threadName=Thread-0i=27
nosynchronized threadName=Thread-0i=28
nosynchronized threadName=Thread-0i=29
nosynchronized threadName=Thread-1i=12
nosynchronized threadName=Thread-0i=30
nosynchronized threadName=Thread-1i=13
nosynchronized threadName=Thread-0i=31
nosynchronized threadName=Thread-1i=14
nosynchronized threadName=Thread-0i=32
nosynchronized threadName=Thread-1i=15
nosynchronized threadName=Thread-0i=33
nosynchronized threadName=Thread-1i=16
nosynchronized threadName=Thread-0i=34
nosynchronized threadName=Thread-1i=17
nosynchronized threadName=Thread-0i=35
nosynchronized threadName=Thread-1i=18
nosynchronized threadName=Thread-0i=36
nosynchronized threadName=Thread-1i=19
nosynchronized threadName=Thread-0i=37
nosynchronized threadName=Thread-0i=38
nosynchronized threadName=Thread-1i=20
nosynchronized threadName=Thread-0i=39
nosynchronized threadName=Thread-1i=21
nosynchronized threadName=Thread-0i=40
nosynchronized threadName=Thread-1i=22
nosynchronized threadName=Thread-0i=41
nosynchronized threadName=Thread-1i=23
nosynchronized threadName=Thread-0i=42
nosynchronized threadName=Thread-1i=24
nosynchronized threadName=Thread-0i=43
nosynchronized threadName=Thread-1i=25
nosynchronized threadName=Thread-0i=44
nosynchronized threadName=Thread-1i=26
nosynchronized threadName=Thread-0i=45
nosynchronized threadName=Thread-1i=27
nosynchronized threadName=Thread-0i=46
nosynchronized threadName=Thread-1i=28
nosynchronized threadName=Thread-0i=47
nosynchronized threadName=Thread-1i=29
nosynchronized threadName=Thread-0i=48
nosynchronized threadName=Thread-1i=30
nosynchronized threadName=Thread-0i=49
nosynchronized threadName=Thread-0i=50
nosynchronized threadName=Thread-1i=31
nosynchronized threadName=Thread-1i=32
nosynchronized threadName=Thread-1i=33
nosynchronized threadName=Thread-1i=34
nosynchronized threadName=Thread-0i=51
nosynchronized threadName=Thread-1i=35
nosynchronized threadName=Thread-0i=52
nosynchronized threadName=Thread-1i=36
nosynchronized threadName=Thread-0i=53
nosynchronized threadName=Thread-0i=54
nosynchronized threadName=Thread-0i=55
nosynchronized threadName=Thread-0i=56
nosynchronized threadName=Thread-0i=57
nosynchronized threadName=Thread-0i=58
nosynchronized threadName=Thread-0i=59
nosynchronized threadName=Thread-0i=60
nosynchronized threadName=Thread-0i=61
nosynchronized threadName=Thread-0i=62
nosynchronized threadName=Thread-1i=37
nosynchronized threadName=Thread-0i=63
nosynchronized threadName=Thread-1i=38
nosynchronized threadName=Thread-0i=64
nosynchronized threadName=Thread-1i=39
nosynchronized threadName=Thread-0i=65
nosynchronized threadName=Thread-1i=40
nosynchronized threadName=Thread-0i=66
nosynchronized threadName=Thread-1i=41
nosynchronized threadName=Thread-0i=67
nosynchronized threadName=Thread-1i=42
nosynchronized threadName=Thread-0i=68
nosynchronized threadName=Thread-1i=43
nosynchronized threadName=Thread-0i=69
nosynchronized threadName=Thread-1i=44
nosynchronized threadName=Thread-0i=70
nosynchronized threadName=Thread-1i=45
nosynchronized threadName=Thread-0i=71
nosynchronized threadName=Thread-1i=46
nosynchronized threadName=Thread-0i=72
nosynchronized threadName=Thread-1i=47
nosynchronized threadName=Thread-1i=48
nosynchronized threadName=Thread-1i=49
nosynchronized threadName=Thread-1i=50
nosynchronized threadName=Thread-1i=51
nosynchronized threadName=Thread-0i=73
nosynchronized threadName=Thread-1i=52
nosynchronized threadName=Thread-0i=74
nosynchronized threadName=Thread-1i=53
nosynchronized threadName=Thread-0i=75
nosynchronized threadName=Thread-1i=54
nosynchronized threadName=Thread-0i=76
nosynchronized threadName=Thread-0i=77
nosynchronized threadName=Thread-0i=78
nosynchronized threadName=Thread-0i=79
nosynchronized threadName=Thread-0i=80
nosynchronized threadName=Thread-1i=55
nosynchronized threadName=Thread-0i=81
nosynchronized threadName=Thread-0i=82
nosynchronized threadName=Thread-1i=56
nosynchronized threadName=Thread-0i=83
nosynchronized threadName=Thread-1i=57
nosynchronized threadName=Thread-1i=58
nosynchronized threadName=Thread-0i=84
nosynchronized threadName=Thread-1i=59
nosynchronized threadName=Thread-0i=85
nosynchronized threadName=Thread-1i=60
nosynchronized threadName=Thread-0i=86
nosynchronized threadName=Thread-1i=61
nosynchronized threadName=Thread-0i=87
nosynchronized threadName=Thread-1i=62
nosynchronized threadName=Thread-0i=88
nosynchronized threadName=Thread-1i=63
nosynchronized threadName=Thread-0i=89
nosynchronized threadName=Thread-1i=64
nosynchronized threadName=Thread-0i=90
nosynchronized threadName=Thread-1i=65
nosynchronized threadName=Thread-0i=91
nosynchronized threadName=Thread-1i=66
nosynchronized threadName=Thread-0i=92
nosynchronized threadName=Thread-1i=67
nosynchronized threadName=Thread-0i=93
nosynchronized threadName=Thread-1i=68
nosynchronized threadName=Thread-0i=94
nosynchronized threadName=Thread-1i=69
nosynchronized threadName=Thread-0i=95
nosynchronized threadName=Thread-1i=70
nosynchronized threadName=Thread-0i=96
nosynchronized threadName=Thread-1i=71
nosynchronized threadName=Thread-0i=97
nosynchronized threadName=Thread-1i=72
nosynchronized threadName=Thread-0i=98
nosynchronized threadName=Thread-1i=73
nosynchronized threadName=Thread-0i=99
nosynchronized threadName=Thread-1i=74
nosynchronized threadName=Thread-0i=100
nosynchronized threadName=Thread-1i=75nosynchronized threadName=Thread-1i=76
synchronized threadName=Thread-0i=1
nosynchronized threadName=Thread-1i=77
synchronized threadName=Thread-0i=2
nosynchronized threadName=Thread-1i=78
synchronized threadName=Thread-0i=3
synchronized threadName=Thread-0i=4
synchronized threadName=Thread-0i=5
nosynchronized threadName=Thread-1i=79
synchronized threadName=Thread-0i=6
nosynchronized threadName=Thread-1i=80
synchronized threadName=Thread-0i=7
nosynchronized threadName=Thread-1i=81
synchronized threadName=Thread-0i=8
synchronized threadName=Thread-0i=9
synchronized threadName=Thread-0i=10
synchronized threadName=Thread-0i=11
synchronized threadName=Thread-0i=12
synchronized threadName=Thread-0i=13
synchronized threadName=Thread-0i=14
synchronized threadName=Thread-0i=15
synchronized threadName=Thread-0i=16
synchronized threadName=Thread-0i=17
synchronized threadName=Thread-0i=18
synchronized threadName=Thread-0i=19
synchronized threadName=Thread-0i=20
synchronized threadName=Thread-0i=21
synchronized threadName=Thread-0i=22
synchronized threadName=Thread-0i=23
nosynchronized threadName=Thread-1i=82
synchronized threadName=Thread-0i=24
nosynchronized threadName=Thread-1i=83
synchronized threadName=Thread-0i=25
nosynchronized threadName=Thread-1i=84
synchronized threadName=Thread-0i=26
nosynchronized threadName=Thread-1i=85
synchronized threadName=Thread-0i=27
nosynchronized threadName=Thread-1i=86
synchronized threadName=Thread-0i=28
nosynchronized threadName=Thread-1i=87
synchronized threadName=Thread-0i=29
nosynchronized threadName=Thread-1i=88
synchronized threadName=Thread-0i=30
synchronized threadName=Thread-0i=31
synchronized threadName=Thread-0i=32
synchronized threadName=Thread-0i=33
nosynchronized threadName=Thread-1i=89
synchronized threadName=Thread-0i=34
nosynchronized threadName=Thread-1i=90
synchronized threadName=Thread-0i=35
nosynchronized threadName=Thread-1i=91
synchronized threadName=Thread-0i=36
nosynchronized threadName=Thread-1i=92
synchronized threadName=Thread-0i=37
nosynchronized threadName=Thread-1i=93
synchronized threadName=Thread-0i=38
nosynchronized threadName=Thread-1i=94
nosynchronized threadName=Thread-1i=95
nosynchronized threadName=Thread-1i=96
nosynchronized threadName=Thread-1i=97
synchronized threadName=Thread-0i=39
nosynchronized threadName=Thread-1i=98
synchronized threadName=Thread-0i=40
nosynchronized threadName=Thread-1i=99
synchronized threadName=Thread-0i=41
nosynchronized threadName=Thread-1i=100synchronized threadName=Thread-0i=42
synchronized threadName=Thread-0i=43
synchronized threadName=Thread-0i=44
synchronized threadName=Thread-0i=45
synchronized threadName=Thread-0i=46
synchronized threadName=Thread-0i=47
synchronized threadName=Thread-0i=48
synchronized threadName=Thread-0i=49
synchronized threadName=Thread-0i=50
synchronized threadName=Thread-0i=51
synchronized threadName=Thread-0i=52
synchronized threadName=Thread-0i=53
synchronized threadName=Thread-0i=54
synchronized threadName=Thread-0i=55
synchronized threadName=Thread-0i=56
synchronized threadName=Thread-0i=57
synchronized threadName=Thread-0i=58
synchronized threadName=Thread-0i=59
synchronized threadName=Thread-0i=60
synchronized threadName=Thread-0i=61
synchronized threadName=Thread-0i=62
synchronized threadName=Thread-0i=63
synchronized threadName=Thread-0i=64
synchronized threadName=Thread-0i=65
synchronized threadName=Thread-0i=66
synchronized threadName=Thread-0i=67
synchronized threadName=Thread-0i=68
synchronized threadName=Thread-0i=69
synchronized threadName=Thread-0i=70
synchronized threadName=Thread-0i=71
synchronized threadName=Thread-0i=72
synchronized threadName=Thread-0i=73
synchronized threadName=Thread-0i=74
synchronized threadName=Thread-0i=75
synchronized threadName=Thread-0i=76
synchronized threadName=Thread-0i=77
synchronized threadName=Thread-0i=78
synchronized threadName=Thread-0i=79
synchronized threadName=Thread-0i=80
synchronized threadName=Thread-0i=81
synchronized threadName=Thread-0i=82
synchronized threadName=Thread-0i=83
synchronized threadName=Thread-0i=84
synchronized threadName=Thread-0i=85
synchronized threadName=Thread-0i=86
synchronized threadName=Thread-0i=87
synchronized threadName=Thread-0i=88
synchronized threadName=Thread-0i=89
synchronized threadName=Thread-0i=90
synchronized threadName=Thread-0i=91
synchronized threadName=Thread-0i=92
synchronized threadName=Thread-0i=93
synchronized threadName=Thread-0i=94
synchronized threadName=Thread-0i=95
synchronized threadName=Thread-0i=96
synchronized threadName=Thread-0i=97
synchronized threadName=Thread-0i=98
synchronized threadName=Thread-0i=99
synchronized threadName=Thread-0i=100
synchronized threadName=Thread-1i=1
synchronized threadName=Thread-1i=2
synchronized threadName=Thread-1i=3
synchronized threadName=Thread-1i=4
synchronized threadName=Thread-1i=5
synchronized threadName=Thread-1i=6
synchronized threadName=Thread-1i=7
synchronized threadName=Thread-1i=8
synchronized threadName=Thread-1i=9
synchronized threadName=Thread-1i=10
synchronized threadName=Thread-1i=11
synchronized threadName=Thread-1i=12
synchronized threadName=Thread-1i=13
synchronized threadName=Thread-1i=14
synchronized threadName=Thread-1i=15
synchronized threadName=Thread-1i=16
synchronized threadName=Thread-1i=17
synchronized threadName=Thread-1i=18
synchronized threadName=Thread-1i=19
synchronized threadName=Thread-1i=20
synchronized threadName=Thread-1i=21
synchronized threadName=Thread-1i=22
synchronized threadName=Thread-1i=23
synchronized threadName=Thread-1i=24
synchronized threadName=Thread-1i=25
synchronized threadName=Thread-1i=26
synchronized threadName=Thread-1i=27
synchronized threadName=Thread-1i=28
synchronized threadName=Thread-1i=29
synchronized threadName=Thread-1i=30
synchronized threadName=Thread-1i=31
synchronized threadName=Thread-1i=32
synchronized threadName=Thread-1i=33
synchronized threadName=Thread-1i=34
synchronized threadName=Thread-1i=35
synchronized threadName=Thread-1i=36
synchronized threadName=Thread-1i=37
synchronized threadName=Thread-1i=38
synchronized threadName=Thread-1i=39
synchronized threadName=Thread-1i=40
synchronized threadName=Thread-1i=41
synchronized threadName=Thread-1i=42
synchronized threadName=Thread-1i=43
synchronized threadName=Thread-1i=44
synchronized threadName=Thread-1i=45
synchronized threadName=Thread-1i=46
synchronized threadName=Thread-1i=47
synchronized threadName=Thread-1i=48
synchronized threadName=Thread-1i=49
synchronized threadName=Thread-1i=50
synchronized threadName=Thread-1i=51
synchronized threadName=Thread-1i=52
synchronized threadName=Thread-1i=53
synchronized threadName=Thread-1i=54
synchronized threadName=Thread-1i=55
synchronized threadName=Thread-1i=56
synchronized threadName=Thread-1i=57
synchronized threadName=Thread-1i=58
synchronized threadName=Thread-1i=59
synchronized threadName=Thread-1i=60
synchronized threadName=Thread-1i=61
synchronized threadName=Thread-1i=62
synchronized threadName=Thread-1i=63
synchronized threadName=Thread-1i=64
synchronized threadName=Thread-1i=65
synchronized threadName=Thread-1i=66
synchronized threadName=Thread-1i=67
synchronized threadName=Thread-1i=68
synchronized threadName=Thread-1i=69
synchronized threadName=Thread-1i=70
synchronized threadName=Thread-1i=71
synchronized threadName=Thread-1i=72
synchronized threadName=Thread-1i=73
synchronized threadName=Thread-1i=74
synchronized threadName=Thread-1i=75
synchronized threadName=Thread-1i=76
synchronized threadName=Thread-1i=77
synchronized threadName=Thread-1i=78
synchronized threadName=Thread-1i=79
synchronized threadName=Thread-1i=80
synchronized threadName=Thread-1i=81
synchronized threadName=Thread-1i=82
synchronized threadName=Thread-1i=83
synchronized threadName=Thread-1i=84
synchronized threadName=Thread-1i=85
synchronized threadName=Thread-1i=86
synchronized threadName=Thread-1i=87
synchronized threadName=Thread-1i=88
synchronized threadName=Thread-1i=89
synchronized threadName=Thread-1i=90
synchronized threadName=Thread-1i=91
synchronized threadName=Thread-1i=92
synchronized threadName=Thread-1i=93
synchronized threadName=Thread-1i=94
synchronized threadName=Thread-1i=95
synchronized threadName=Thread-1i=96
synchronized threadName=Thread-1i=97
synchronized threadName=Thread-1i=98
synchronized threadName=Thread-1i=99
synchronized threadName=Thread-1i=100
    

          用synchronized（this）来锁定当前对象，通过语句块来进行同步。
     
       
        

  

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