jvm重新排序/能见度效应测试

Question

在编写一些java文章时,我试图在多线程环境中的非同步对象构造的情况下重现重新排序.构建重型对象时没有同步/挥发性/韵母和其他线程在构造函数调用后立即访问它的情况.这是我尝试的代码:

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 +
                    '}';
        }
    }
} 

- 但到目前为止没有效果,我尝试了不同的2,4和8核心Intel/AMD PC与Windows,运行测试几个小时 - 没有重新排序效果 - System.err.printf("观察者%s看到.. .") - 未调用,static sharedArray [randomIndex]引用始终包含完全构造的值.

怎么了？如何重现这个？

Answer 1

这是一篇不错的文章，应该展示 x86 上的重新排序（这是一项壮举，因为 x86 内存模型几乎“安全”）：

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“分配后但构造前存储对象引用”，因为构造函数可能会抛出异常，因此需要恢复引用。由于给定的保证，某些处理器可能会重新排序，但没有英特尔兼容的处理器。