我有一个方法，调用该方法的 2 个 synchronize block ，并提供了代码，
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，并从每个线程中调用 compute 方法。下面提供了代码片段，
Thread t1 = new Thread(() -> compute());
Thread t2 = new Thread(() -> compute());
t1.start();
t2.start();
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
根据提供的代码，我有三个问题:
当线程调用compute方法时，它是否可以访问内部的两个synchronized方法并执行增量操作？
如果一个线程访问 for 循环，它会完成 for 循环还是在竞争之前将其保留以与其他线程共享？
如果是这种情况，我假设另一个线程处于等待阶段并且不做太多事情。这是正确的吗？
为了讨论，这是我运行后得到的结果:
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，当线程确实获得访问权限时，它会同时拥有两个synchronized 方法。当控制从一个线程切换到另一线程时，计数器值会更新并被记住。
最佳答案
将会发生的情况是，两个线程都调用 compute() 方法，进入 for 循环，并在第一次调用 add() 时结束。
两者之一将先于另一个到达，因此他将进入 synchronized 方法。当他在该方法中时，第二个线程可能会到达 add() 调用。这是一个可能，因为你无法说出哪个线程在哪个时间点位于哪个位置。 JVM 决定哪些线程可以在什么时间执行操作是未定义的。但我们假设第二个线程到达 add() 调用并且现在想要执行该方法。它不能这样做，因为它是同步并且另一个线程当前已经在调用它。所以线程进入等待状态。第一个线程完成对 add() 的调用。现在，第二个线程可能会进入 add()，执行调用，退出 add() 并进入 addAgain() 。现在，第一个线程正在等待。
这可能听起来令人困惑，而这正是我想要达到的目的:
你无法说出哪个线程何时调用哪个方法。 JVM 可能决定让第一个线程首先执行这两个方法的 100 次调用，然后第二个线程执行相同的操作。它可能会决定他们称它们为交替。现实情况可能介于两者之间。在另一个线程完成初始化之前，两个线程之一甚至可能不会进入compute()方法。你说不出来。这一切都取决于您的系统、处理器的数量、发生的代码和 CPU 优化、正在使用的算法调度程序等等。
一旦你进行了多线程处理，发生什么事情就脱离了你的掌控。通过调用join，您可以等待任务完成。但没有确定的方法可以保证在 1 毫秒、2 毫秒或 250 毫秒内会出现这种情况。
所以你的问题:
- When a thread calls the compute method, does it get access to both of the synchronized methods inside and execute the increment operation?
不，没有。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。
- If a thread gets access to the for loop, does it complete the for loop or leave it before the competition to share with the other thread?
你不能说。它既不保证完成循环，也不保证在完成之前被中断。
- If its the case, I assume the other thread is in the wait phase and don't do much. Is this correct?
如果线程尝试访问已被另一个线程访问的同步方法，则该线程将处于等待状态。
关于java - 多线程中的同步块(synchronized block)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/49316209/