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软引用

在许多的开源组件之中，往往会使用软引用作为缓存组件出现，其最大的特点在于:

内存不足时回收，内存空间充足时不回收。

如果想要实现软引用，则必须有一个单独的类(java.lang.ref.SoftReference，其是java.lang.ref.Reference的一个子类)来实现控制：

这个类的方法如下：

  • 构造：
/**
     * Creates a new soft reference that refers to the given object.  The new
     * reference is not registered with any queue.
     *
     * @param referent object the new soft reference will refer to
     */
    public SoftReference(T referent) {
        super(referent);
        this.timestamp = clock;
    }
  • 取出数据：
/**
     * Returns this reference object's referent.  If this reference object has
     * been cleared, either by the program or by the garbage collector, then
     * this method returns <code>null</code>.
     *
     * @return   The object to which this reference refers, or
     *           <code>null</code> if this reference object has been cleared
     */
    public T get() {
        T o = super.get();
        if (o != null && this.timestamp != clock)
            this.timestamp = clock;
        return o;
    }

观察软引用：

示例代码：

public class TestDemo3 {

    public static void main(String[] args) {
        Object obj = new Object();
        //软引用
        SoftReference<Object> ref = new SoftReference<>(obj);
        //断开链接
        obj = null;
        Runtime.getRuntime().gc();
        System.out.println(ref.get());
    }
}

输出下列结果：

java.lang.Object@60e53b93

表明：

ref这个软引用并没有被回收，没有被回收的原因是此时此刻内存不紧张。

为了观察内存紧张时的软引用回收情况，现在通过JVM配置将此类所分配到的内存“变的紧张”，类中代码如下：

public class TestDemo3 {

    public static void main(String[] args) {
        Object obj = new Object();
        String str = "hello";

        //断开链接
        obj = null;
        //软引用
        SoftReference<Object> ref = new SoftReference<>(obj);
        try{
            for (int i = 0; i < 100; i++) {
                str = str + i;
                str.intern();
            }
        }catch (Throwable throwable){

        }
//        Runtime.getRuntime().gc();
        System.out.println(ref.get() + "     ###########################");
    }
}

再执行下列命令：

java -Xmx10m -Xms10m -XX:+PrintGCDetails TestDemo3

此时会输出以下结果:

/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java -Xmx10m -Xms10m -XX:+PrintGCDetails "-javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=65026:/Applications/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/lib/tools.jar:/Users/yangyi/LintCode/out/production/LintCode TestDemo3
objc[19074]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java (0x10fb5e4c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x10fbea4e0). One of the two will be used. Which one is undefined.
null     ###########################
Heap
 PSYoungGen      total 2560K, used 1751K [0x00000007bfd00000, 0x00000007c0000000, 0x00000007c0000000)
  eden space 2048K, 85% used [0x00000007bfd00000,0x00000007bfeb5cb8,0x00000007bff00000)
  from space 512K, 0% used [0x00000007bff80000,0x00000007bff80000,0x00000007c0000000)
  to   space 512K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007bff80000)
 ParOldGen       total 7168K, used 0K [0x00000007bf600000, 0x00000007bfd00000, 0x00000007bfd00000)
  object space 7168K, 0% used [0x00000007bf600000,0x00000007bf600000,0x00000007bfd00000)
 Metaspace       used 3153K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 350K, capacity 388K, committed 512K, reserved 1048576K

Process finished with exit code 0

可以看到控制台输出的null。

如果此时内存空间充足，那么对象将不会被回收；如果空间不充足，则会进行回收处理。