所谓的引用队列指的就是保存那些准备被回收的对象。很多的时候所有的对象的回收扫描，都是从根对象开始的。那么对于整个GC而言，如果要想确定哪些对象可以被回收，我们就必须确定好引用的强度，这个也就是所谓的引用路径的设置。
如果现在要找到对象5，那么很明显1找到5属于“强+软”，那么2找到5属于“强+弱”。软引用要比弱引用保存的强。所以这个时候实际上对于对象的引用而言，如果要进行引用的关联判断，那么久必须找到强关联，那么为了避免非强关联或者强引用对象所带来的内存问题，所以提供有一个叫做引用队列的概念，如果在创建软引用或者弱引用类型的时候使用了引用队列的方式，则这个对象被税收之后会自动保存在引用队列之中。
使用引用队列：
示例代码如下：
public class TestDemo5 {
public static void main(String[] args) {
Object obj = new Object();
ReferenceQueue<Object> queue = new ReferenceQueue<>();
WeakReference<Object> ref = new WeakReference<Object>(obj, queue);
System.out.println(queue.poll());
}
}
输出结果如下：
/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=57389:/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 TestDemo5
objc[1379]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java (0x1062784c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x1063044e0). One of the two will be used. Which one is undefined.
null
Heap
PSYoungGen total 2560K, used 1669K [0x00000007bfd00000, 0x00000007c0000000, 0x00000007c0000000)
eden space 2048K, 81% used [0x00000007bfd00000,0x00000007bfea14c8,0x00000007bff00000)
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 3177K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 357K, capacity 388K, committed 512K, reserved 1048576K
Process finished with exit code 0
加入GC的操作，此时因为是弱引用，所以一定会被回收，因为GC的操作有延迟，所以这个让其延迟2000ms执行：
public class TestDemo5 {
public static void main(String[] args) throws Exception{
Object obj = new Object();
ReferenceQueue<Object> queue = new ReferenceQueue<>();
WeakReference<Object> ref = new WeakReference<Object>(obj, queue);
System.out.println(queue.poll());
obj = null;
Runtime.getRuntime().gc();
Thread.sleep(2000);
System.out.println(queue.poll());
}
}
输出信息如下：
/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=57441:/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 TestDemo5
objc[1398]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java (0x10965a4c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x1096e64e0). One of the two will be used. Which one is undefined.
null
[GC (System.gc()) [PSYoungGen: 1627K->496K(2560K)] 1627K->528K(9728K), 0.0040205 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
[Full GC (System.gc()) [PSYoungGen: 496K->0K(2560K)] [ParOldGen: 32K->391K(7168K)] 528K->391K(9728K), [Metaspace: 3147K->3147K(1056768K)], 0.0104091 secs] [Times: user=0.03 sys=0.00, real=0.01 secs]
java.lang.ref.WeakReference@60e53b93
Heap
PSYoungGen total 2560K, used 229K [0x00000007bfd00000, 0x00000007c0000000, 0x00000007c0000000)
eden space 2048K, 11% used [0x00000007bfd00000,0x00000007bfd39560,0x00000007bff00000)
from space 512K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007bff80000)
to space 512K, 0% used [0x00000007bff80000,0x00000007bff80000,0x00000007c0000000)
ParOldGen total 7168K, used 391K [0x00000007bf600000, 0x00000007bfd00000, 0x00000007bfd00000)
object space 7168K, 5% used [0x00000007bf600000,0x00000007bf661ea0,0x00000007bfd00000)
Metaspace used 3315K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 369K, capacity 388K, committed 512K, reserved 1048576K
Process finished with exit code 0
这种引用队列主要是做一些被回收对象的控制。