jvm重排/可见度效应测试 [英] jvm reordering/visibility effect test

问题描述

在编写一些java文章时，我试图重现重新排序在多线程环境中的非同步对象构造的情况下。
当重构对象被构造为w / o synchonization / volatiles / finals和其他线程在构造函数调用后立即访问它的情况。
这里是我尝试的代码：

While writing some java article I'm trying to reproduce re-ordering in case of unsynchronized object costruction in multithreaded environment. The case when a heavy object is constructed w/o synchonization/volatiles/finals and other threads get access to it right after constructor call. Here is the code I try:

public class ReorderingTest {
    static SomeObject<JPanel>[] sharedArray = new SomeObject[100];

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            String name = "watcher" + i;
            new Thread(new Watcher(name)).start();
            System.out.printf("watcher %s started!%n", name);
        }
    }

    static class Watcher implements Runnable {
        private String name;

        Watcher(String name) {
            this.name = name;
        }

        public void run() {
            while (true) {
                int randomIndex = (int) (Math.random() * sharedArray.length);
                SomeObject<JPanel> item = sharedArray[randomIndex];
                if (item == null) {
                    //System.out.printf("sharedArray[%s]=null%n", randomIndex);
                    double r = 1 + Math.random() * 1000;
                    sharedArray[randomIndex] = new SomeObject<JPanel>(
                            new JPanel(), UUID.randomUUID().toString(), r, (float)r * 33, (long)r);
                } else {
                    //System.out.printf("sharedArray[%s]=<obj>!%n", randomIndex);
                    if (item.value == null ||
                            (item.stringField == null) ||
                            (item.doubleField == 0) ||
                            (item.floatField == 0) ||
                            (item.longField == 0)
                            ) {
                        System.err.printf("watcher %s sees default values: %s!%n", name, item);
                    } else {
                        // fully initialized! run new construction process
                        double r = 1 + Math.random() * 1000;
                        sharedArray[randomIndex] = new SomeObject<JPanel>(
                                new JPanel(), UUID.randomUUID().toString(), r, (float)r * 37, (long)r);

                    }
                }
                /*try {
                    TimeUnit.NANOSECONDS.sleep(randomIndex);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }*/
            }
        }
    }

    static class SomeObject<V> {
        V value;
        String stringField;
        double doubleField;
        float floatField;
        long longField;

        SomeObject(V value, String stringField, double doubleField, float floatField, long longField) {
            this.value = value;
            this.stringField = stringField;
            this.doubleField = doubleField;
            this.floatField = floatField;
            this.longField = longField;
        }

        @Override
        public String toString() {
            return "SomeObject{" +
                    "value=" + value == null ? "null" : "<obj>" +
                    ", stringField='" + stringField + '\'' +
                    ", doubleField=" + doubleField +
                    ", floatField=" + floatField +
                    ", longField=" + longField +
                    '}';
        }
    }
} 

，我试过在不同的2,4和8核心英特尔/ AMD电脑与Windows，跑了几个小时的测试 - 没有重排序效果 - System.err.printf（watcher％s看到...） - 不是调用，静态的sharedArray [randomIndex]引用总是包含完全构造的值。

-But no effect so far, I've tried on different 2,4 and 8 core Intel/AMD PCs with windows, ran test for a few hours - no reordering effect - System.err.printf("watcher %s sees ...") - is not called, static sharedArray[randomIndex] reference always contains fully constructed values.

有什么问题？如何重现这个？

What's wrong? How to reproduce this?

推荐答案

这里是一篇很好的文章，应该在x86上显示重新排序（相当不错，因为x86内存模型是 ）：

Here is a nice article which should show reordering on x86 (quite a feat because the x86 memory model is pretty much "safe"):

http://bartoszmilewski.wordpress.com/2008/11/05/who-ordered-memory-fences-on-an-x86/

您的示例不会显示重新排序。编译器不会重新排序为在分配后但在构造之前存储对象引用，因为构造函数可能抛出，因此引用将需要恢复。一些处理器可能会重新排序，但没有英特尔兼容的，由于给定的保证。

Your example will not show reordering. The compiler will not reorder to "store object reference after allocation but before construction" because the constructor could throw and thus the reference would need to be reverted. Some Processors may reorder this, but no intel compatible ones, due to the given guaranties.

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