多线程中的同步块

时间：2018-03-16 08:32:36

标签： java multithreading

我有一个方法调用方法的2 synchronize块并提供代码，

    private static int count1 = 0;
    private static int count2 = 0;

    public synchronized static void add() {
    count1++;
    System.out.println("T name: " + Thread.currentThread().getName()
    + "count1 = " + count1);
}

public synchronized static void addAgain() {
    count2++;
    System.out.println("T name: " + Thread.currentThread().getName()
            + "count1 = " + count2);
}

    public static void compute() {
        for(int i=0;i<100;++i) {
            add();
            addAgain();
        }
    }

这里我们定义了2个线程，并从每个线程调用compute方法。代码段在下面提供，

        Thread t1 = new Thread(() -> compute());
        Thread t2 = new Thread(() -> compute());

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

提供代码后，我有三个问题：

  1. 当一个线程调用compute方法时，是否可以访问内部的synchronized个方法并执行增量操作？

  2. 如果一个线程可以访问for循环，它是否完成for循环或者在竞争之前将其保留以与其他线程共享？

  3. 如果是这种情况，我假设另一个线程处于等待阶段而且做得不多。这是对的吗？

    4. 关于讨论的问题，这是我在跑步后获得的结果：

        Thread name: Thread-0  count1 value = 1
Thread name: Thread-1  count1 value = 2
Thread name: Thread-1 count2 value = 1
Thread name: Thread-1  count1 value = 3
Thread name: Thread-1 count2 value = 2
Thread name: Thread-1  count1 value = 4
Thread name: Thread-1 count2 value = 3
Thread name: Thread-1  count1 value = 5
Thread name: Thread-1 count2 value = 4
Thread name: Thread-1  count1 value = 6
Thread name: Thread-1 count2 value = 5
Thread name: Thread-1  count1 value = 7
Thread name: Thread-1 count2 value = 6
Thread name: Thread-1  count1 value = 8
Thread name: Thread-1 count2 value = 7
Thread name: Thread-1  count1 value = 9
Thread name: Thread-1 count2 value = 8
Thread name: Thread-1  count1 value = 10
Thread name: Thread-1 count2 value = 9
Thread name: Thread-1  count1 value = 11
Thread name: Thread-1 count2 value = 10
Thread name: Thread-1  count1 value = 12
Thread name: Thread-1 count2 value = 11
Thread name: Thread-1  count1 value = 13
Thread name: Thread-1 count2 value = 12
Thread name: Thread-1  count1 value = 14
Thread name: Thread-1 count2 value = 13
Thread name: Thread-1  count1 value = 15
Thread name: Thread-1 count2 value = 14
Thread name: Thread-1  count1 value = 16
Thread name: Thread-1 count2 value = 15
Thread name: Thread-1  count1 value = 17
Thread name: Thread-1 count2 value = 16
Thread name: Thread-1  count1 value = 18
Thread name: Thread-1 count2 value = 17
Thread name: Thread-1  count1 value = 19
Thread name: Thread-1 count2 value = 18
Thread name: Thread-1  count1 value = 20
Thread name: Thread-0 count2 value = 19
Thread name: Thread-0  count1 value = 21
Thread name: Thread-0 count2 value = 20
Thread name: Thread-0  count1 value = 22
Thread name: Thread-0 count2 value = 21
Thread name: Thread-0  count1 value = 23
Thread name: Thread-0 count2 value = 22
Thread name: Thread-0  count1 value = 24
Thread name: Thread-0 count2 value = 23
Thread name: Thread-0  count1 value = 25
Thread name: Thread-0 count2 value = 24
Thread name: Thread-0  count1 value = 26
Thread name: Thread-0 count2 value = 25
Thread name: Thread-0  count1 value = 27
Thread name: Thread-0 count2 value = 26
Thread name: Thread-0  count1 value = 28
Thread name: Thread-0 count2 value = 27
Thread name: Thread-0  count1 value = 29
Thread name: Thread-0 count2 value = 28
Thread name: Thread-0  count1 value = 30
Thread name: Thread-0 count2 value = 29
Thread name: Thread-0  count1 value = 31
Thread name: Thread-0 count2 value = 30
Thread name: Thread-0  count1 value = 32
Thread name: Thread-0 count2 value = 31
Thread name: Thread-0  count1 value = 33
Thread name: Thread-0 count2 value = 32
Thread name: Thread-0  count1 value = 34
Thread name: Thread-0 count2 value = 33
Thread name: Thread-0  count1 value = 35
Thread name: Thread-0 count2 value = 34
Thread name: Thread-0  count1 value = 36
Thread name: Thread-0 count2 value = 35
Thread name: Thread-0  count1 value = 37
Thread name: Thread-0 count2 value = 36
Thread name: Thread-0  count1 value = 38
Thread name: Thread-0 count2 value = 37
Thread name: Thread-0  count1 value = 39
Thread name: Thread-0 count2 value = 38
Thread name: Thread-0  count1 value = 40
Thread name: Thread-0 count2 value = 39
Thread name: Thread-0  count1 value = 41
Thread name: Thread-0 count2 value = 40
Thread name: Thread-0  count1 value = 42
Thread name: Thread-0 count2 value = 41
Thread name: Thread-0  count1 value = 43
Thread name: Thread-0 count2 value = 42
Thread name: Thread-0  count1 value = 44
Thread name: Thread-0 count2 value = 43
Thread name: Thread-0  count1 value = 45
Thread name: Thread-0 count2 value = 44
Thread name: Thread-0  count1 value = 46
Thread name: Thread-0 count2 value = 45
Thread name: Thread-0  count1 value = 47
Thread name: Thread-0 count2 value = 46
Thread name: Thread-0  count1 value = 48
Thread name: Thread-0 count2 value = 47
Thread name: Thread-0  count1 value = 49
Thread name: Thread-0 count2 value = 48
Thread name: Thread-0  count1 value = 50
Thread name: Thread-0 count2 value = 49
Thread name: Thread-0  count1 value = 51
Thread name: Thread-0 count2 value = 50
Thread name: Thread-0  count1 value = 52
Thread name: Thread-0 count2 value = 51
Thread name: Thread-0  count1 value = 53
Thread name: Thread-0 count2 value = 52
Thread name: Thread-0  count1 value = 54
Thread name: Thread-0 count2 value = 53
Thread name: Thread-0  count1 value = 55
Thread name: Thread-0 count2 value = 54
Thread name: Thread-0  count1 value = 56
Thread name: Thread-0 count2 value = 55
Thread name: Thread-0  count1 value = 57
Thread name: Thread-0 count2 value = 56
Thread name: Thread-0  count1 value = 58
Thread name: Thread-0 count2 value = 57
Thread name: Thread-0  count1 value = 59
Thread name: Thread-0 count2 value = 58
Thread name: Thread-0  count1 value = 60
Thread name: Thread-0 count2 value = 59
Thread name: Thread-0  count1 value = 61
Thread name: Thread-0 count2 value = 60
Thread name: Thread-0  count1 value = 62
Thread name: Thread-0 count2 value = 61
Thread name: Thread-0  count1 value = 63
Thread name: Thread-0 count2 value = 62
Thread name: Thread-0  count1 value = 64
Thread name: Thread-0 count2 value = 63
Thread name: Thread-0  count1 value = 65
Thread name: Thread-0 count2 value = 64
Thread name: Thread-0  count1 value = 66
Thread name: Thread-0 count2 value = 65
Thread name: Thread-0  count1 value = 67
Thread name: Thread-0 count2 value = 66
Thread name: Thread-0  count1 value = 68
Thread name: Thread-0 count2 value = 67
Thread name: Thread-0  count1 value = 69
Thread name: Thread-0 count2 value = 68
Thread name: Thread-0  count1 value = 70
Thread name: Thread-0 count2 value = 69
Thread name: Thread-0  count1 value = 71
Thread name: Thread-0 count2 value = 70
Thread name: Thread-0  count1 value = 72
Thread name: Thread-0 count2 value = 71
Thread name: Thread-0  count1 value = 73
Thread name: Thread-0 count2 value = 72
Thread name: Thread-0  count1 value = 74
Thread name: Thread-0 count2 value = 73
Thread name: Thread-0  count1 value = 75
Thread name: Thread-0 count2 value = 74
Thread name: Thread-0  count1 value = 76
Thread name: Thread-0 count2 value = 75
Thread name: Thread-0  count1 value = 77
Thread name: Thread-0 count2 value = 76
Thread name: Thread-0  count1 value = 78
Thread name: Thread-0 count2 value = 77
Thread name: Thread-0  count1 value = 79
Thread name: Thread-0 count2 value = 78
Thread name: Thread-0  count1 value = 80
Thread name: Thread-0 count2 value = 79
Thread name: Thread-0  count1 value = 81
Thread name: Thread-0 count2 value = 80
Thread name: Thread-0  count1 value = 82
Thread name: Thread-0 count2 value = 81
Thread name: Thread-0  count1 value = 83
Thread name: Thread-0 count2 value = 82
Thread name: Thread-0  count1 value = 84
Thread name: Thread-0 count2 value = 83
Thread name: Thread-0  count1 value = 85
Thread name: Thread-0 count2 value = 84
Thread name: Thread-0  count1 value = 86
Thread name: Thread-0 count2 value = 85
Thread name: Thread-0  count1 value = 87
Thread name: Thread-0 count2 value = 86
Thread name: Thread-0  count1 value = 88
Thread name: Thread-0 count2 value = 87
Thread name: Thread-0  count1 value = 89
Thread name: Thread-0 count2 value = 88
Thread name: Thread-0  count1 value = 90
Thread name: Thread-0 count2 value = 89
Thread name: Thread-0  count1 value = 91
Thread name: Thread-0 count2 value = 90
Thread name: Thread-0  count1 value = 92
Thread name: Thread-0 count2 value = 91
Thread name: Thread-0  count1 value = 93
Thread name: Thread-0 count2 value = 92
Thread name: Thread-0  count1 value = 94
Thread name: Thread-0 count2 value = 93
Thread name: Thread-0  count1 value = 95
Thread name: Thread-0 count2 value = 94
Thread name: Thread-0  count1 value = 96
Thread name: Thread-0 count2 value = 95
Thread name: Thread-0  count1 value = 97
Thread name: Thread-0 count2 value = 96
Thread name: Thread-0  count1 value = 98
Thread name: Thread-0 count2 value = 97
Thread name: Thread-0  count1 value = 99
Thread name: Thread-0 count2 value = 98
Thread name: Thread-0  count1 value = 100
Thread name: Thread-0 count2 value = 99
Thread name: Thread-0  count1 value = 101
Thread name: Thread-0 count2 value = 100
Thread name: Thread-0  count1 value = 102
Thread name: Thread-0 count2 value = 101
Thread name: Thread-0  count1 value = 103
Thread name: Thread-0 count2 value = 102
Thread name: Thread-0  count1 value = 104
Thread name: Thread-0 count2 value = 103
Thread name: Thread-0  count1 value = 105
Thread name: Thread-0 count2 value = 104
Thread name: Thread-0  count1 value = 106
Thread name: Thread-0 count2 value = 105
Thread name: Thread-0  count1 value = 107
Thread name: Thread-0 count2 value = 106
Thread name: Thread-0  count1 value = 108
Thread name: Thread-0 count2 value = 107
Thread name: Thread-0  count1 value = 109
Thread name: Thread-0 count2 value = 108
Thread name: Thread-0  count1 value = 110
Thread name: Thread-0 count2 value = 109
Thread name: Thread-0  count1 value = 111
Thread name: Thread-0 count2 value = 110
Thread name: Thread-0  count1 value = 112
Thread name: Thread-0 count2 value = 111
Thread name: Thread-0  count1 value = 113
Thread name: Thread-0 count2 value = 112
Thread name: Thread-0  count1 value = 114
Thread name: Thread-0 count2 value = 113
Thread name: Thread-0  count1 value = 115
Thread name: Thread-0 count2 value = 114
Thread name: Thread-0  count1 value = 116
Thread name: Thread-0 count2 value = 115
Thread name: Thread-0  count1 value = 117
Thread name: Thread-0 count2 value = 116
Thread name: Thread-0  count1 value = 118
Thread name: Thread-0 count2 value = 117
Thread name: Thread-0  count1 value = 119
Thread name: Thread-0 count2 value = 118
Thread name: Thread-1 count2 value = 119
Thread name: Thread-1  count1 value = 120
Thread name: Thread-1 count2 value = 120
Thread name: Thread-1  count1 value = 121
Thread name: Thread-1 count2 value = 121
Thread name: Thread-1  count1 value = 122
Thread name: Thread-1 count2 value = 122
Thread name: Thread-1  count1 value = 123
Thread name: Thread-1 count2 value = 123
Thread name: Thread-1  count1 value = 124
Thread name: Thread-1 count2 value = 124
Thread name: Thread-1  count1 value = 125
Thread name: Thread-1 count2 value = 125
Thread name: Thread-1  count1 value = 126
Thread name: Thread-1 count2 value = 126
Thread name: Thread-1  count1 value = 127
Thread name: Thread-1 count2 value = 127
Thread name: Thread-1  count1 value = 128
Thread name: Thread-1 count2 value = 128
Thread name: Thread-1  count1 value = 129
Thread name: Thread-1 count2 value = 129
Thread name: Thread-1  count1 value = 130
Thread name: Thread-1 count2 value = 130
Thread name: Thread-1  count1 value = 131
Thread name: Thread-1 count2 value = 131
Thread name: Thread-1  count1 value = 132
Thread name: Thread-1 count2 value = 132
Thread name: Thread-1  count1 value = 133
Thread name: Thread-1 count2 value = 133
Thread name: Thread-1  count1 value = 134
Thread name: Thread-1 count2 value = 134
Thread name: Thread-1  count1 value = 135
Thread name: Thread-1 count2 value = 135
Thread name: Thread-1  count1 value = 136
Thread name: Thread-1 count2 value = 136
Thread name: Thread-1  count1 value = 137
Thread name: Thread-1 count2 value = 137
Thread name: Thread-1  count1 value = 138
Thread name: Thread-1 count2 value = 138
Thread name: Thread-1  count1 value = 139
Thread name: Thread-1 count2 value = 139
Thread name: Thread-1  count1 value = 140
Thread name: Thread-1 count2 value = 140
Thread name: Thread-1  count1 value = 141
Thread name: Thread-1 count2 value = 141
Thread name: Thread-1  count1 value = 142
Thread name: Thread-1 count2 value = 142
Thread name: Thread-1  count1 value = 143
Thread name: Thread-1 count2 value = 143
Thread name: Thread-1  count1 value = 144
Thread name: Thread-1 count2 value = 144
Thread name: Thread-1  count1 value = 145
Thread name: Thread-1 count2 value = 145
Thread name: Thread-1  count1 value = 146
Thread name: Thread-1 count2 value = 146
Thread name: Thread-1  count1 value = 147
Thread name: Thread-1 count2 value = 147
Thread name: Thread-1  count1 value = 148
Thread name: Thread-1 count2 value = 148
Thread name: Thread-1  count1 value = 149
Thread name: Thread-1 count2 value = 149
Thread name: Thread-1  count1 value = 150
Thread name: Thread-1 count2 value = 150
Thread name: Thread-1  count1 value = 151
Thread name: Thread-1 count2 value = 151
Thread name: Thread-1  count1 value = 152
Thread name: Thread-1 count2 value = 152
Thread name: Thread-1  count1 value = 153
Thread name: Thread-1 count2 value = 153
Thread name: Thread-1  count1 value = 154
Thread name: Thread-1 count2 value = 154
Thread name: Thread-1  count1 value = 155
Thread name: Thread-1 count2 value = 155
Thread name: Thread-1  count1 value = 156
Thread name: Thread-1 count2 value = 156
Thread name: Thread-1  count1 value = 157
Thread name: Thread-1 count2 value = 157
Thread name: Thread-1  count1 value = 158
Thread name: Thread-1 count2 value = 158
Thread name: Thread-1  count1 value = 159
Thread name: Thread-1 count2 value = 159
Thread name: Thread-1  count1 value = 160
Thread name: Thread-1 count2 value = 160
Thread name: Thread-1  count1 value = 161
Thread name: Thread-1 count2 value = 161
Thread name: Thread-1  count1 value = 162
Thread name: Thread-1 count2 value = 162
Thread name: Thread-1  count1 value = 163
Thread name: Thread-1 count2 value = 163
Thread name: Thread-1  count1 value = 164
Thread name: Thread-1 count2 value = 164
Thread name: Thread-1  count1 value = 165
Thread name: Thread-1 count2 value = 165
Thread name: Thread-1  count1 value = 166
Thread name: Thread-1 count2 value = 166
Thread name: Thread-1  count1 value = 167
Thread name: Thread-1 count2 value = 167
Thread name: Thread-1  count1 value = 168
Thread name: Thread-1 count2 value = 168
Thread name: Thread-1  count1 value = 169
Thread name: Thread-1 count2 value = 169
Thread name: Thread-1  count1 value = 170
Thread name: Thread-1 count2 value = 170
Thread name: Thread-1  count1 value = 171
Thread name: Thread-1 count2 value = 171
Thread name: Thread-1  count1 value = 172
Thread name: Thread-1 count2 value = 172
Thread name: Thread-1  count1 value = 173
Thread name: Thread-1 count2 value = 173
Thread name: Thread-1  count1 value = 174
Thread name: Thread-1 count2 value = 174
Thread name: Thread-1  count1 value = 175
Thread name: Thread-1 count2 value = 175
Thread name: Thread-1  count1 value = 176
Thread name: Thread-1 count2 value = 176
Thread name: Thread-1  count1 value = 177
Thread name: Thread-1 count2 value = 177
Thread name: Thread-1  count1 value = 178
Thread name: Thread-1 count2 value = 178
Thread name: Thread-1  count1 value = 179
Thread name: Thread-1 count2 value = 179
Thread name: Thread-1  count1 value = 180
Thread name: Thread-1 count2 value = 180
Thread name: Thread-1  count1 value = 181
Thread name: Thread-1 count2 value = 181
Thread name: Thread-1  count1 value = 182
Thread name: Thread-1 count2 value = 182
Thread name: Thread-1  count1 value = 183
Thread name: Thread-1 count2 value = 183
Thread name: Thread-1  count1 value = 184
Thread name: Thread-1 count2 value = 184
Thread name: Thread-1  count1 value = 185
Thread name: Thread-1 count2 value = 185
Thread name: Thread-1  count1 value = 186
Thread name: Thread-1 count2 value = 186
Thread name: Thread-1  count1 value = 187
Thread name: Thread-1 count2 value = 187
Thread name: Thread-1  count1 value = 188
Thread name: Thread-1 count2 value = 188
Thread name: Thread-1  count1 value = 189
Thread name: Thread-1 count2 value = 189
Thread name: Thread-1  count1 value = 190
Thread name: Thread-1 count2 value = 190
Thread name: Thread-1  count1 value = 191
Thread name: Thread-1 count2 value = 191
Thread name: Thread-1  count1 value = 192
Thread name: Thread-1 count2 value = 192
Thread name: Thread-1  count1 value = 193
Thread name: Thread-1 count2 value = 193
Thread name: Thread-1  count1 value = 194
Thread name: Thread-1 count2 value = 194
Thread name: Thread-1  count1 value = 195
Thread name: Thread-1 count2 value = 195
Thread name: Thread-1  count1 value = 196
Thread name: Thread-1 count2 value = 196
Thread name: Thread-1  count1 value = 197
Thread name: Thread-1 count2 value = 197
Thread name: Thread-1  count1 value = 198
Thread name: Thread-1 count2 value = 198
Thread name: Thread-1  count1 value = 199
Thread name: Thread-1 count2 value = 199
Thread name: Thread-1  count1 value = 200
Thread name: Thread-1 count2 value = 200
Count1=200  Count2=200

    因此，当一个线程获得访问权限时，它会同时拥有两个synchronized方法。当控制从一个thrad切换到另一个时，计数器值被更新并被记住。

1 个答案:

答案 0 :(得分：3)

将会发生两个线程调用compute()方法，进入for循环并在第一次调用add()时结束。

其中一个将在另一个之前，因此他将进入synchronized方法。当他在方法中时，第二个线程可能会到达add()电话。这是一种可能，因为你不能说哪个线程在哪个时间点。未定义JVM决定哪些线程可以在什么时间执行操作。但是我们假设第二个线程到达add()调用，现在想要执行该方法。它不能这样做synchronized而另一个线程当前已经在调用它。所以线程进入等待状态。第一个帖子完成了他对add()的调用。现在，第二个主题可能会进入add()，进行通话，退回add()并输入addAgain()。这使得第一个线程等待。

这可能听起来令人困惑，而这正是我想要的：

你不能说哪个线程调用什么方法。 JVM可能决定让你的第一个线程先完成两个方法的100次调用，然后第二个线程也会这样做。它可能会决定他们交替称呼它们。现实可能介于两者之间。在另一个线程完成初始化之前，两个线程中的一个甚至可能不会进入compute()方法。你不能说。这完全取决于您的系统，处理器的数量，代码和CPU优​​化，使用的算法调度程序等等。

一旦你进行多线程处理，当发生的事情发生时，它就会失控。通过调用join，您可以等待任务完成。但是有没有确定性方法可以保证在1ms，2ms或250ms内出现这种情况。

所以你的问题：

  
      
  1. 当线程调用compute方法时，它是否可以访问内部的两个同步方法并执行增量   操作
    2.   

不，不。通过调用add，它可以访问此方法。它不会自动访问第二种方法。

  
      
  1. 如果一个线程可以访问for循环，它是否完成for循环或者在竞争之前将其保留以与其他线程共享？
    2.   

你不能说。它既不能保证完成循环也不能在完成之前中断。

  
      
  1. 如果是这种情况，我假设另一个线程处于等待阶段但没有做太多。这是对的吗？
    2.   

如果一个线程在另一个线程已经访问过的时候尝试访问synchronized方法，它将处于等待状态。

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