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faoxis
Asked:2020-04-15 05:02:20 +0000 UTC2020-04-15 05:02:20 +0000 UTC

更新 SoftReference 类中的时间戳字段

  • 772

该类的源代码SoftReference有这个字段：

/**
 * Timestamp updated by each invocation of the get method.  The VM may use
 * this field when selecting soft references to be cleared, but it is not
 * required to do so.
 */
private long timestamp;

根据该方法的文档，每次调用都get必须更新timestamp. 但它没有，我不知道为什么。作为一个实验，我写了以下类：

import java.lang.ref.SoftReference;
import java.lang.reflect.Field;

public class SoftReferenceTest {
    public static void main(String[] args) throws Exception {

        SoftReference<StringBuilder> softReference = new SoftReference<>(new StringBuilder());
        long timestamp = getTimestamp(softReference);

        for (int i = 0; i < 10; i++) {
            checkTimestampChange(softReference, timestamp);
        }

    }

    private static void checkTimestampChange(SoftReference softReference, long timestampBefore) throws Exception {
        softReference.get();
        Thread.sleep(100);
        assert timestampBefore == getTimestamp(softReference);
    }


    private static long getTimestamp(Object object) throws Exception {
        Field timestampField = SoftReference.class.getDeclaredField("timestamp");
        timestampField.setAccessible(true);
        long timestamp = timestampField.getLong(object);
        timestampField.setAccessible(false);

        return timestamp;
    }
}

代码运行成功，没有错误。我不明白为什么。

java

1 个回答

  1. Best Answer

    你看过源代码SoftReference吗？clock根据文档，在 GC 调用期间有一个字段会更新。

    /**
 * Timestamp clock, updated by the garbage collector
 */
static private long clock;

    和方法代码get

    public T get() {
    T o = super.get();
    if (o != null && this.timestamp != clock)
        this.timestamp = clock;
    return o;
}

    您不会更改timestamp，因为在循环执行期间没有调用垃圾收集器。

    如果你明确地调用它，那么它会timestamp被更新：

    public static void main(String[] args) throws Exception {

    SoftReference<StringBuilder> softReference = new SoftReference<>(new StringBuilder());
    long timestamp = getTimestamp(softReference);

    for (int i = 0; i < 10; i++) {
        checkTimestampChange(softReference, timestamp);
        System.gc();
    }

}

private static void checkTimestampChange(SoftReference softReference, long timestampBefore) throws Exception {
    softReference.get();
    Thread.sleep(100);
    System.out.println(timestampBefore == getTimestamp(softReference));
}

    真的

    错误的

    错误的

    错误的

    ...

    如果您删除System.gc()上面代码中的调用，它显然会显示在您的案例中true（但不能保证）

    • 4

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