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我创建了一个程序来检查是否@sun.misc.Contended生效。这个想法是，当@Contended生效时，注释类中的字段偏移量会更大。

如果我指定了-XX:-RestrictContended标志，我可以看到 OpenJDK 上偏移量的预期差异。我在 OpenJ9 11 (jdk-11.0.1+13, Eclipse OpenJ9 VM-11.0.1) 上没有看到任何区别。

OpenJDK 输出是

readOnly: 12
writeOnly: 16
----
readOnly: 12
writeOnly: 144

OpenJ9 的输出是

readOnly: 8
writeOnly: 12
----
readOnly: 8
writeOnly: 12

该程序是

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;

public class Main {
    public static class Baseline {
        int readOnly;
        int writeOnly;
    }

    public static class Contended {
        int readOnly;

        @sun.misc.Contended
        int writeOnly;
    }

    public static void main(String[] args) throws Exception {
        Baseline b = new Baseline();
        Contended s = new Contended();

        printOffsets(b);
        System.out.println("----");
        printOffsets(s);
    }

    // https://blog.hazelcast.com/using-sun-misc-unsafe-in-java-9/
    @SuppressWarnings("restriction")
    private static Unsafe getUnsafe() throws NoSuchFieldException, IllegalAccessException {
        Field singleoneInstanceField = Unsafe.class.getDeclaredField("theUnsafe");
        singleoneInstanceField.setAccessible(true);
        return (Unsafe) singleoneInstanceField.get(null);
    }

    // http://mishadoff.com/blog/java-magic-part-4-sun-dot-misc-dot-unsafe/
    public static void printOffsets(Object o) throws Exception {
        Unsafe u = getUnsafe();
        Class c = o.getClass();
        for (Field f : c.getDeclaredFields()) {
            if ((f.getModifiers() & Modifier.STATIC) == 0) {
                printOffset(u, f);
            }
        }
    }

    public static void printOffset(Unsafe u, Field f) {
            long offset = u.objectFieldOffset(f);
            System.out.println(f.getName() + ": " + offset);
    }
}
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1 回答 1

2

目前，OpenJ9 仅支持@Contended类级别。

于 2018-11-16T16:44:19.670 回答