最近在看netty的时候看到直接内存的相关概念，为了更详细的了解一下具体原理，搜到了一篇不错的文章 http://lovestblog.cn/blog/2015/05/12/direct-buffer/
因为文章中又涉及到PhantomReference的概念，又为了更详细的了解一下具体原理，又搜到了另一篇不错的文章 https://blog.****.net/xlinsist/article/details/57089288
这些概念很多文章中都有讲解，下面主要是针对各个Reference的具体实践（体现在最后的代码部分，各个实验都有相关注释，实验环境及所列源码均为jdk1.8.0_172）
Reference抽象类有两个构造方法，referent是必须参数，queue可选
Reference被类加载器加载初始化后会启动一个后台线程，这个线程的作用是将jvm传给它的Reference们加入到它们自己的queue中
新标签页查看大图
加入queue的Reference就可以被持有queue的对象进行相应的逻辑处理了，例如WeakHashMap会把不新鲜的对象给清除掉
Cleaner与DirectByteBuffer的部分源码
下面是根据各个Reference的定义做的一些具体实验（实验结果表明，Weak和Cleaner一遇到gc同时只剩下它们引用它们的referent的时候，这些referent一定会被清除掉）
实验记得加上main注释上的启动参数
package xyz.fz.test; import sun.misc.Cleaner; import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference; public class ReferenceTest { private static final int MB = 1024 * 1024; private static ReferenceQueue<Object> queue = new ReferenceQueue<>(); private static class BigBaby {
// just hold memory
private byte[] weight = new byte[20 * MB]; BigBaby() {
}
} private static class MyCleanerRunner implements Runnable {
private String somethingYouWant; MyCleanerRunner(String something) {
this.somethingYouWant = something;
} @Override
public void run() {
System.out.println(somethingYouWant);
}
} // -Xms64M -Xmx64M -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps
public static void main(String[] args) {
System.out.println("============= softReferenceBigBabyGcNotWork =============");
System.gc();
softReferenceBigBabyGcNotWork();
System.out.println("============= softReferenceBigBabyGc =============");
System.gc();
softReferenceBigBabyGc();
System.out.println("============= weakReferenceBigBabyGc =============");
System.gc();
weakReferenceBigBabyGc();
System.out.println("============= phantomReferenceBigBabyGc =============");
System.gc();
phantomReferenceBigBabyGc();
System.out.println("============= cleanerBigBabyGc =============");
System.gc();
cleanerBigBabyGc();
System.out.println("============= queueFifoTest =============");
System.gc();
queueFifoTest();
} private static void softReferenceBigBabyGcNotWork() {
// 内存充足
// 软持有对象置空
// 主动gc
// 软持有对象没有被清除
BigBaby bigBaby = new BigBaby();
SoftReference<BigBaby> softReference = new SoftReference<>(bigBaby, queue);
bigBaby = null;
gcAndWait();
System.out.println("SoftReference's Referent: " + softReference.get());
printQueue();
} private static void softReferenceBigBabyGc() {
// 内存充足
// 软持有对象置空
// 创建新对象（导致内存不足）
// 主动（或被动）gc
// 软持有对象被清除，该软引用加入引用队列
BigBaby bigBaby = new BigBaby();
SoftReference<BigBaby> softReference = new SoftReference<>(bigBaby, queue);
bigBaby = null;
BigBaby bigBaby2 = new BigBaby();
BigBaby bigBaby3 = new BigBaby();
gcAndWait();
System.out.println("SoftReference's Referent: " + softReference.get());
printQueue();
} private static void weakReferenceBigBabyGc() {
// 内存充足
// 弱持有对象置空
// 主动gc
// 弱持有对象被清除，弱引用加入引用队列
BigBaby bigBaby = new BigBaby();
WeakReference<BigBaby> weakReference = new WeakReference<>(bigBaby, queue);
bigBaby = null;
gcAndWait();
System.out.println("WeakReference's Referent: " + weakReference.get());
printQueue();
} private static void phantomReferenceBigBabyGc() {
// 内存充足
// 幻持有对象置空
// 主动gc
// 幻持有对象没有被清除（内存充足的原因？），幻引用加入引用队列
BigBaby bigBaby = new BigBaby();
PhantomReference<BigBaby> phantomReference = new PhantomReference<>(bigBaby, queue);
bigBaby = null;
gcAndWait();
printQueue();
} private static void cleanerBigBabyGc() {
// 内存充足
// Cleaner持有对象置空
// 主动gc
// Cleaner持有对象被清除，Cleaner的runner被执行 /*
Cleaner是一个特殊的幻引用，
虽然它的构造中也有引用队列，但这个引用队列是个假引用队列，因为它从来不会被使用，仅仅作为幻引用的必要参数而已，
真正使用的是其定义的runner，
在持有对象被清除后runner得到执行
*/ /*
DirectByteBuffer用于分配堆外内存，
其中就有一个属性为cleaner，
并且该cleaner的持有对象就是其自身(DirectByteBuffer)，
也就是说当这个DirectByteBuffer被gc回收之后，
cleaner中的runner方法将得到执行（对堆外内存进行回收）
*/
BigBaby bigBaby = new BigBaby();
Cleaner cleaner = Cleaner.create(bigBaby, new MyCleanerRunner("I'm Cleaner's runner. I can do what you want to do."));
bigBaby = null;
gcAndWait();
} private static void queueFifoTest() {
/*
从queue的名字会误认为是先进先出的队列，但是从实现和实验中可以看出他其实是后进先出
*/
BigBaby bigBaby = new BigBaby();
WeakReference<BigBaby> weakReference = new WeakReference<>(bigBaby, queue);
System.out.println(weakReference);
bigBaby = null;
gcAndWait(); bigBaby = new BigBaby();
WeakReference<BigBaby> weakReference2 = new WeakReference<>(bigBaby, queue);
System.out.println(weakReference2);
bigBaby = null;
gcAndWait(); bigBaby = new BigBaby();
WeakReference<BigBaby> weakReference3 = new WeakReference<>(bigBaby, queue);
System.out.println(weakReference3);
bigBaby = null;
gcAndWait(); printQueue();
} private static void printQueue() {
Object o;
int size = 0;
while ((o = queue.poll()) != null) {
System.out.println("Reference: " + o);
size++;
}
System.out.println("Reference Queue Size: " + size);
} private static void gcAndWait() {
System.gc();
try {
System.out.println("gc waiting ...");
Thread.sleep(1000L);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
执行结果如下：
============= softReferenceBigBabyGcNotWork =============
2018-07-18T17:24:34.874+0800: 0.239: [GC (System.gc()) [PSYoungGen: 3994K->1112K(18944K)] 3994K->1120K(62976K), 0.0012344 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:34.876+0800: 0.240: [Full GC (System.gc()) [PSYoungGen: 1112K->0K(18944K)] [ParOldGen: 8K->1015K(44032K)] 1120K->1015K(62976K), [Metaspace: 3467K->3467K(1056768K)], 0.0048045 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:34.890+0800: 0.255: [GC (System.gc()) [PSYoungGen: 327K->96K(18944K)] 21823K->21591K(62976K), 0.0004161 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:34.891+0800: 0.255: [Full GC (System.gc()) [PSYoungGen: 96K->0K(18944K)] [ParOldGen: 21495K->21389K(44032K)] 21591K->21389K(62976K), [Metaspace: 3469K->3469K(1056768K)], 0.0098622 secs] [Times: user=0.03 sys=0.02, real=0.01 secs]
gc waiting ...
SoftReference's Referent: xyz.fz.test.ReferenceTest$BigBaby@3b07d329
Reference Queue Size: 0
============= softReferenceBigBabyGc =============
2018-07-18T17:24:35.901+0800: 1.266: [GC (System.gc()) [PSYoungGen: 655K->64K(18944K)] 22045K->21453K(62976K), 0.0006612 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:35.902+0800: 1.267: [Full GC (System.gc()) [PSYoungGen: 64K->0K(18944K)] [ParOldGen: 21389K->900K(44032K)] 21453K->900K(62976K), [Metaspace: 3470K->3470K(1056768K)], 0.0119403 secs] [Times: user=0.06 sys=0.00, real=0.01 secs]
2018-07-18T17:24:35.927+0800: 1.292: [GC (Allocation Failure) [PSYoungGen: 0K->32K(18944K)] 41860K->41892K(62976K), 0.0012161 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:35.928+0800: 1.293: [Full GC (Ergonomics) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 41860K->41860K(44032K)] 41892K->41860K(62976K), [Metaspace: 3471K->3471K(1056768K)], 0.0043798 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:35.933+0800: 1.297: [GC (Allocation Failure) [PSYoungGen: 0K->0K(18944K)] 41860K->41860K(62976K), 0.0006057 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:35.933+0800: 1.298: [Full GC (Allocation Failure) [PSYoungGen: 0K->0K(18944K)] [ParOldGen: 41860K->21362K(44032K)] 41860K->21362K(62976K), [Metaspace: 3471K->3471K(1056768K)], 0.0130772 secs] [Times: user=0.06 sys=0.00, real=0.01 secs]
2018-07-18T17:24:35.948+0800: 1.313: [GC (System.gc()) [PSYoungGen: 0K->0K(18944K)] 41842K->41842K(62976K), 0.0005524 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:35.949+0800: 1.313: [Full GC (System.gc()) [PSYoungGen: 0K->0K(18944K)] [ParOldGen: 41842K->41842K(44032K)] 41842K->41842K(62976K), [Metaspace: 3471K->3471K(1056768K)], 0.0023415 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
gc waiting ...
SoftReference's Referent: null
Reference: java.lang.ref.SoftReference@41629346
Reference Queue Size: 1
============= weakReferenceBigBabyGc =============
2018-07-18T17:24:36.951+0800: 2.316: [GC (System.gc()) [PSYoungGen: 327K->32K(18944K)] 42170K->41874K(62976K), 0.0007930 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:36.952+0800: 2.317: [Full GC (System.gc()) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 41842K->882K(44032K)] 41874K->882K(62976K), [Metaspace: 3471K->3471K(1056768K)], 0.0072817 secs] [Times: user=0.00 sys=0.02, real=0.01 secs]
2018-07-18T17:24:36.962+0800: 2.326: [GC (System.gc()) [PSYoungGen: 327K->64K(18944K)] 21690K->21426K(62976K), 0.0004395 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:36.962+0800: 2.327: [Full GC (System.gc()) [PSYoungGen: 64K->0K(18944K)] [ParOldGen: 21362K->883K(44032K)] 21426K->883K(62976K), [Metaspace: 3471K->3471K(1056768K)], 0.0074164 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
gc waiting ...
WeakReference's Referent: null
Reference: java.lang.ref.WeakReference@404b9385
Reference Queue Size: 1
============= phantomReferenceBigBabyGc =============
2018-07-18T17:24:37.970+0800: 3.335: [GC (System.gc()) [PSYoungGen: 327K->64K(18944K)] 1210K->947K(62976K), 0.0004446 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:37.971+0800: 3.335: [Full GC (System.gc()) [PSYoungGen: 64K->0K(18944K)] [ParOldGen: 883K->883K(44032K)] 947K->883K(62976K), [Metaspace: 3471K->3471K(1056768K)], 0.0077789 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2018-07-18T17:24:37.981+0800: 3.346: [GC (System.gc()) [PSYoungGen: 327K->96K(18944K)] 21691K->21459K(62976K), 0.0004417 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:37.981+0800: 3.346: [Full GC (System.gc()) [PSYoungGen: 96K->0K(18944K)] [ParOldGen: 21363K->21363K(44032K)] 21459K->21363K(62976K), [Metaspace: 3472K->3472K(1056768K)], 0.0042990 secs] [Times: user=0.06 sys=0.00, real=0.01 secs]
gc waiting ...
Reference: java.lang.ref.PhantomReference@6d311334
Reference Queue Size: 1
============= cleanerBigBabyGc =============
2018-07-18T17:24:38.986+0800: 4.351: [GC (System.gc()) [PSYoungGen: 327K->32K(18944K)] 21691K->21395K(62976K), 0.0004979 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:38.987+0800: 4.351: [Full GC (System.gc()) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 21363K->883K(44032K)] 21395K->883K(62976K), [Metaspace: 3472K->3472K(1056768K)], 0.0077382 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2018-07-18T17:24:38.998+0800: 4.363: [GC (System.gc()) [PSYoungGen: 327K->160K(18944K)] 21691K->21523K(62976K), 0.0005646 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:38.999+0800: 4.364: [Full GC (System.gc()) [PSYoungGen: 160K->0K(18944K)] [ParOldGen: 21363K->884K(44032K)] 21523K->884K(62976K), [Metaspace: 3482K->3482K(1056768K)], 0.0077882 secs] [Times: user=0.06 sys=0.00, real=0.01 secs]
gc waiting ...
I'm Cleaner's runner. I can do what you want to do.
============= queueFifoTest =============
2018-07-18T17:24:40.007+0800: 5.372: [GC (System.gc()) [PSYoungGen: 655K->32K(18944K)] 1540K->924K(62976K), 0.0004796 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:40.008+0800: 5.372: [Full GC (System.gc()) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 892K->884K(44032K)] 924K->884K(62976K), [Metaspace: 3487K->3487K(1056768K)], 0.0079891 secs] [Times: user=0.05 sys=0.00, real=0.01 secs]
java.lang.ref.WeakReference@682a0b20
2018-07-18T17:24:40.018+0800: 5.383: [GC (System.gc()) [PSYoungGen: 327K->32K(18944K)] 21692K->21396K(62976K), 0.0005765 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:40.019+0800: 5.383: [Full GC (System.gc()) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 21364K->884K(44032K)] 21396K->884K(62976K), [Metaspace: 3487K->3487K(1056768K)], 0.0080978 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
gc waiting ...
java.lang.ref.WeakReference@3d075dc0
2018-07-18T17:24:41.029+0800: 6.394: [GC (System.gc()) [PSYoungGen: 327K->32K(18944K)] 21692K->21396K(62976K), 0.0006605 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-07-18T17:24:41.030+0800: 6.395: [Full GC (System.gc()) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 21364K->884K(44032K)] 21396K->884K(62976K), [Metaspace: 3487K->3487K(1056768K)], 0.0083753 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
gc waiting ...
java.lang.ref.WeakReference@214c265e
2018-07-18T17:24:42.041+0800: 7.406: [GC (System.gc()) [PSYoungGen: 327K->32K(18944K)] 21692K->21396K(62976K), 0.0006599 secs] [Times: user=0.00 sys=0.02, real=0.00 secs]
2018-07-18T17:24:42.042+0800: 7.407: [Full GC (System.gc()) [PSYoungGen: 32K->0K(18944K)] [ParOldGen: 21364K->884K(44032K)] 21396K->884K(62976K), [Metaspace: 3487K->3487K(1056768K)], 0.0102298 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
gc waiting ...
Reference: java.lang.ref.WeakReference@214c265e
Reference: java.lang.ref.WeakReference@3d075dc0
Reference: java.lang.ref.WeakReference@682a0b20
Reference Queue Size: 3
Heap
PSYoungGen total 18944K, used 655K [0x00000000feb00000, 0x0000000100000000, 0x0000000100000000)
eden space 16384K, 4% used [0x00000000feb00000,0x00000000feba3f90,0x00000000ffb00000)
from space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000)
to space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000)
ParOldGen total 44032K, used 884K [0x00000000fc000000, 0x00000000feb00000, 0x00000000feb00000)
object space 44032K, 2% used [0x00000000fc000000,0x00000000fc0dd328,0x00000000feb00000)
Metaspace used 3494K, capacity 4564K, committed 4864K, reserved 1056768K
class space used 377K, capacity 388K, committed 512K, reserved 1048576K