Java 编程思想的这个 OrnamentalGarden 例子为什么是这种打印结果？

这是 java 编程思想的一个小例子，大概意思，五个 Runnable 对象会对自己加锁（但对自己加锁，因为没有别人和它本身冲突，所以没有意义），还有个 Count 对象，它也有同步方法，五个线程都会调到它，自然，只有它的同步方法才是有意义的。

import java.util.concurrent.*;
import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;

import static net.mindview.util.Print.*;

class Count {
    private int count = 0;
    private Random rand = new Random(47);
    // Remove the synchronized keyword to see counting fail:
    public synchronized int increment() {
        int temp = count;
        System.out.println(Thread.currentThread().getName()+" befor yield");
        if(rand.nextBoolean()) // Yield half the time
            Thread.yield();
        System.out.println(Thread.currentThread().getName()+" after yield");
        return (count = ++temp);
    }
    public synchronized int value() { return count; }
}

class Entrance implements Runnable {
    private static Count count = new Count();
    private static List<Entrance> entrances =
            new ArrayList<Entrance>();
    private int number = 0;
    // Doesn't need synchronization to read:
    private final int id;
    private static volatile boolean canceled = false;
    // Atomic operation on a volatile field:
    public static void cancel() { canceled = true; }
    public Entrance(int id) {
        this.id = id;
        // Keep this task in a list. Also prevents
        // garbage collection of dead tasks:
        
        entrances.add(this);
    }
    public void run() {
        while(!canceled) {
            synchronized(this) {
                ++number;
            }
            print(this + " Total: " + count.increment());
            try {
                TimeUnit.MILLISECONDS.sleep(100);
            } catch(InterruptedException e) {
                print("sleep interrupted");
            }
        }
        print("Stopping " + this);
    }
    public synchronized int getValue() { return number; }
    public String toString() {
        return "Entrance " + Thread.currentThread().getName() + ": " + getValue();
    }
    public static int getTotalCount() {
        return count.value();
    }
    public static int sumEntrances() {
        int sum = 0;
        for(Entrance entrance : entrances)
            sum += entrance.getValue();
        return sum;
    }
}

public class OrnamentalGarden {
    public static void main(String[] args) throws Exception {
        AtomicInteger a = new AtomicInteger();
        ExecutorService exec = new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                60L, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>(),new ThreadFactory(){ public Thread newThread(Runnable r) {
            return new Thread(r, "pool_" + a.getAndIncrement());
        }});
        for(int i = 0; i < 5; i++)
            exec.execute(new Entrance(i));
        // Run for a while, then stop and collect the data:
        TimeUnit.SECONDS.sleep(3);
        Entrance.cancel();
        exec.shutdown();
        if(!exec.awaitTermination(250, TimeUnit.MILLISECONDS))
            print("Some tasks were not terminated!");
        print("Total: " + Entrance.getTotalCount());
        print("Sum of Entrances: " + Entrance.sumEntrances());
    }
}

可能好多人看到这里直接略过了，但我看完这个例子确实似懂非懂的。https://paste.ubuntu.com/p/B7GQMcMhv7/ ，这是原版代码，为了看到简单的线程名，稍微改了下。执行完后，打印结果是：

pool_0 befor yield
pool_0 after yield
pool_2 befor yield
pool_2 after yield
pool_4 befor yield
pool_4 after yield
pool_3 befor yield
pool_3 after yield
pool_1 befor yield
pool_1 after yield
Entrance pool_0: 1 Total: 1
Entrance pool_3: 1 Total: 4   为啥这里直接是 4 了？
Entrance pool_1: 1 Total: 5
Entrance pool_2: 1 Total: 2
Entrance pool_4: 1 Total: 3
pool_0 befor yield
pool_0 after yield

如上，为啥那里直接是 4 了？那个 4 是 count 对象的同步方法，按理说应该接下来打印 2 啊。所以这个程序是想说明，1 过了直接打印 4，是因为线程调度吗？作者想说明什么啊

PS：有时候感觉 Java 编程思想的例子好恶心。。。

etoah

2020-04-23 13:26:13 +08:00

这个问题最主要的就 <print(this + " Total: " + count.increment());> 这句话

前面的同步块其实没什么用. 现在假设 5 个线程都走完同步块, 同时到达 <print(this + " Total: " + count.increment());>

先将这个语句分成两个
int countValue = count.increment();
print(this + "Total:" + countValue);

因为同时有 5 个线程在跑我们将上面的代码复制 5 份 (第一行标识代码行号十位数代表线程 id)

pool_0 线程:
01 int countValue00 = count.increment();
02 print(this + "Total:" + countValue00);

pool_1 线程:
11 int countValue01 = count.increment();
12 print(this + "Total:" + countValue01);

pool_2 线程:
21 int countValue02 = count.increment();
22 print(this + "Total:" + countValue02);

pool_3 线程:
31 int countValue03 = count.increment();
32 print(this + "Total:" + countValue03);

pool_4 线程:
41 int countValue04 = count.increment();
42 print(this + "Total:" + countValue04);

分别代表每一条线程执行的代码

从输出结果看 cpu 执行的顺序是
01 21 41 31 11
02 32 12 22 42

第一行执行后 countValue00 - 05 的值分别是
线程 pool_id: 0 2 4 3 1 // 取 cpu 执行顺序的十位数
countValue : 1 2 3 4 5 // 按执行顺序递增

第二行的顺序
线程 pool_id: 0 3 1 2 4
输出 countValue: 1 4 5 2 3

是不是就对上了

主要是 <print(this + " Total: " + count.increment());> 这条语句其实是不是一个原子操作