KeepAliveSoftReference.java

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/*
 * @test KeepAliveSoftReference
 * @summary This test case uses a java.lang.ref.SoftReference referencing a class instance to keep a class alive.
 * @requires vm.opt.final.ClassUnloading
 * @modules java.base/jdk.internal.misc
 * @library /test/lib
 * @library classes
 * @build sun.hotspot.WhiteBox test.Empty
 * @run driver jdk.test.lib.helpers.ClassFileInstaller sun.hotspot.WhiteBox
 * @run main/othervm -Xbootclasspath/a:. -Xmn8m -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI KeepAliveSoftReference
 */

import java.lang.ref.SoftReference;
import sun.hotspot.WhiteBox;
import jdk.test.lib.classloader.ClassUnloadCommon;

/**
 * Test that verifies that classes are not unloaded when specific types of references are kept to them.
 */
public class KeepAliveSoftReference {
  private static final String className = "test.Empty";
  private static final WhiteBox wb = WhiteBox.getWhiteBox();

  public static void main(String... args) throws Exception {
    ClassLoader cl = ClassUnloadCommon.newClassLoader();
    Class<?> c = cl.loadClass(className);
    Object o = c.newInstance();
    SoftReference<Object> sr = new SoftReference(o);
    o = null; c = null; cl = null;

    {
        boolean isAlive = wb.isClassAlive(className);
        System.out.println("testSoftReference (1) alive: " + isAlive);
        boolean cleared = (sr.get() == null);
        boolean shouldBeAlive = !cleared;
        ClassUnloadCommon.failIf(isAlive != shouldBeAlive, "" + isAlive + " != " + shouldBeAlive);
    }

    ClassUnloadCommon.triggerUnloading();

    {
        boolean isAlive = wb.isClassAlive(className);
        System.out.println("testSoftReference (2) alive: " + isAlive);
        boolean cleared = (sr.get() == null);
        boolean shouldBeAlive = !cleared;
        ClassUnloadCommon.failIf(isAlive != shouldBeAlive, "" + isAlive + " != " + shouldBeAlive);
    }
    sr.clear();
    ClassUnloadCommon.triggerUnloading();

    {
        boolean isAlive = wb.isClassAlive(className);
        System.out.println("testSoftReference (3) alive: " + isAlive);
        boolean cleared = (sr.get() == null);
        boolean shouldBeAlive = !cleared;
        ClassUnloadCommon.failIf(isAlive != shouldBeAlive, "" + isAlive + " != " + shouldBeAlive);
    }
  }
}