关于JVM垃圾回收的java.lang.ref.Finalizer问题
作者：影࿐ེ
JVM垃圾回收的java.lang.ref.Finalizer
基础知识
Shallow Size
- 对象自身占用的内存大小，不包括它引用的对象
Retained Size
- Retained Size=当前对象大小+当前对象可直接或间接引用到的对象的大小总和
可达性分析算法：
基本思路是通过一系列成为GC ROOTS 的对象作为起点，当一个对象到 GC ROOTS 没有任何相连，证明此对象是不可达，即被判断为可回收的对象。
之后的过程是：
被标记不可达的对象以后，进行第一次标记，和第一次筛选，条件是该对象 有没有必要执行finalize方法
没有必要执行finalize方法的情况是
1.finalize已经执行过（finalize 方法只会被执行一次）
2.该对象没有重写finalize方法
如果要执行finalize方法，该对象进入一个F-Queue队列，稍后有 一个优先级为8的 finalizer线程来执行（注意：如果一个对象在 finalize 方法中运行缓慢，将会导致队列后的其他对象永远等待，严重时将会导致系统崩溃）
GC对队列中进行第二次标记，如果在执行finalize方法的时候将自己和GC ROOTS关联上，该对象即可逃离回收，否则，被回收掉
重要
对象是在已经被GC识别为是垃圾后才丢到Queue中的，在queue中依然占用内存
引用对象的类型
引用对象的类型第一篇已说过，分为以下类型
强引用>软引用>弱引用>虚引用，
涉及到的相关类：
- 软引用:softReference
- 弱引用:weekRefernce
- 虚引用:pathomReference
但还有一个特殊的引用：FinalReference类，该类也是JVM调优的常见的调优场景之一。
要了解FinalReference,先了解final()
final,finally和finalize()的区别？
前两者我相信大多数人都能答出，但finalize()虽然也有人答出，但可能只是硬搬面试题答案。
1.final：修饰词，作用于成员变量，方法和类。
- 成员变量，基本数据类型表示该成员变量不可变更数值，对象表示该对象引用地址不可改变。
- 方法，表示该方法不可重载
- 类，表示该类不可被继承
2.finally：是异常处理机制的一部分，表示总是执行这段代码，先执行异常，后执行finally
@Slf4j
public class FinallyDemo {
public static void main(String[] args) {
try {
int i = 1 / 0;
log.info("1/0");
} catch (Exception e) {
log.info("执行异常");
} finally {
log.info("执行finally片段");
}
}
}
// 打印结果：
// 10:36:09.777 [main] INFO com.gc.demo.finalize.finlly.FinallyDemo - 执行异常
// 10:36:09.780 [main] INFO com.gc.demo.finalize.finlly.FinallyDemo - 执行finally片段
3.finalize():是object的一个方法。简简单单的一句话，我想大多数人懂得，但其实我们应该继续深入。
Finalize()
Object源码
public class Object {
private static native void registerNatives();
static {
registerNatives();
}
.......
@Deprecated(since="9")
protected void finalize() throws Throwable { }
}
上述代码可以看出：finalize()已被弃用，不建议使用
为什么finalize()已被弃用，不建议使用？
原因：容易导致堆内存溢出。
为什么不直接删除呢？
1. 兼容老旧项目
2. 是GC回收对象前，对该对象生前（被回收前）必须执行的逻辑业务，保证程序正常运行。例如：FileInputStream，源码如下
public class FileInputStream extends InputStream{
......
static class AltFinalizer {
private final FileInputStream fis;
AltFinalizer(FileInputStream fis) {
this.fis = fis;
}
@Override
@SuppressWarnings("deprecation")
protected final void finalize() {
try {
if ((fis.fd != null) && (fis.fd != FileDescriptor.in)) {
/* if fd is shared, the references in FileDescriptor
* will ensure that finalizer is only called when
* safe to do so. All references using the fd have
* become unreachable. We can call close()
*/
fis.close();
}
} catch (IOException ioe) {
// ignore
}
}
}
FileInputStream类的close()方法大家都不陌生，用于关闭流输入，但他被写到finalize()方法内。
原理：
JVM启动时，检测所有对象，是否实现了finalize()方法，如果实现了，就会该对象标记为finalizer类，且交由finalizerThread线程管理，当GC回收时，finalizerThread线程就会处理finalizer类，检测是否有执行finalize()方法，如果有，则可以把finalizer从线程中去除，对应的对象就可以被回收；如果没有，则一直交由finalizerThread线程管理。
详细流程
JVM启动时，检测对象是否实现了finalize()方法，检测方法：判断has_finalizer_flag和registerfinalizerrsaltInit字段
A对象实现了finalize()方法，就会把A对象的finalize()函数注册到finalizerthread线程和referencequeue队列中，注册完毕后，且称该函数注册的B对象称为Finalizer类，指向原有的A对象。
GC回收垃圾时，启动FinalizerThread线程，检测Finalizer类对应的A对象的finalize()方法是否已执行，已调用，则可以把Finalizer类去除，回收Finalizer类，从而回收A对象；如果没有调用，则继续交由FinalizerThread线程管理，不回收。
从上面原因引申出：为什么堆内存溢出？
GC回收时，用户线程、GC回收线程和FinalizerThread线程都在运行，但由于FinalizerThread线程优先度比GC回收线程和用户线程低，所以导致回收速度比对象创建速度慢，最终导致堆内存溢出。
示例程序：
JDK11、G1垃圾收集器
JVM设置
-Xms10m -Xmx10m -Xlog:ref*=debug -Xlog:gc:gc.log -Xlog:gc+heap=trace -verbose:gc -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=E:\mycode\gc-demo\a.dump
public class FinalizeGcDemo {
private static class GcDemo {
private byte[] content = new byte[1024 * 1024];
@Override
protected void finalize() throws Throwable {
System.out.println("执行了finalize方法");
}
}
public static void main(String[] args) {
for (int i = 0; i < 10000; i++) {
GcDemo gcDemo = new GcDemo();
}
}
}
日志如下：
[0.453s][debug][gc,heap ] GC(23) Heap after GC invocations=21 (full 13): garbage-first heap total 10240K, used 10123K [0x00000000ff600000, 0x0000000100000000)
[0.453s][debug][gc,heap ] GC(23) region size 1024K, 0 young (0K), 0 survivors (0K)
[0.453s][debug][gc,heap ] GC(23) Metaspace used 6220K, capacity 6395K, committed 6528K, reserved 1056768K
[0.453s][debug][gc,heap ] GC(23) class space used 538K, capacity 606K, committed 640K, reserved 1048576K
[0.453s][info ][gc ] GC(23) Pause Full (G1 Evacuation Pause) 9M->9M(10M) 5.420ms
[0.455s][debug][gc,heap ] GC(24) Heap before GC invocations=21 (full 13): garbage-first heap total 10240K, used 10123K [0x00000000ff600000, 0x0000000100000000)
[0.455s][debug][gc,heap ] GC(24) region size 1024K, 0 young (0K), 0 survivors (0K)
[0.455s][debug][gc,heap ] GC(24) Metaspace used 6220K, capacity 6395K, committed 6528K, reserved 1056768K
[0.455s][debug][gc,heap ] GC(24) class space used 538K, capacity 606K, committed 640K, reserved 1048576K
[0.455s][debug][gc,ref ] GC(24) Skipped phase1 of Reference Processing due to unavailable references
[0.455s][debug][gc,ref ] GC(24) Skipped phase2 of Reference Processing due to unavailable references
[0.455s][debug][gc,ref ] GC(24) Skipped phase3 of Reference Processing due to unavailable references
[0.455s][debug][gc,ref ] GC(24) Skipped phase4 of Reference Processing due to unavailable references
[0.455s][debug][gc,phases,ref ] GC(24) Reference Processing: 0.0ms
[0.455s][debug][gc,phases,ref ] GC(24) Reconsider SoftReferences: 0.0ms
[0.455s][debug][gc,phases,ref ] GC(24) SoftRef (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) Notify Soft/WeakReferences: 0.0ms
[0.455s][debug][gc,phases,ref ] GC(24) SoftRef (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) WeakRef (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) FinalRef (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) Total (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) Notify and keep alive finalizable: 0.0ms
[0.455s][debug][gc,phases,ref ] GC(24) FinalRef (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) Notify PhantomReferences: 0.0ms
[0.455s][debug][gc,phases,ref ] GC(24) PhantomRef (ms): skipped
[0.455s][debug][gc,phases,ref ] GC(24) SoftReference:
[0.455s][debug][gc,phases,ref ] GC(24) Discovered: 0
[0.455s][debug][gc,phases,ref ] GC(24) Cleared: 0
[0.455s][debug][gc,phases,ref ] GC(24) WeakReference:
[0.455s][debug][gc,phases,ref ] GC(24) Discovered: 0
[0.455s][debug][gc,phases,ref ] GC(24) Cleared: 0
[0.455s][debug][gc,phases,ref ] GC(24) FinalReference:
[0.455s][debug][gc,phases,ref ] GC(24) Discovered: 0
[0.455s][debug][gc,phases,ref ] GC(24) Cleared: 0
[0.455s][debug][gc,phases,ref ] GC(24) PhantomReference:
[0.455s][debug][gc,phases,ref ] GC(24) Discovered: 0
[0.455s][debug][gc,phases,ref ] GC(24) Cleared: 0
[0.455s][info ][gc,heap ] GC(24) Eden regions: 0->0(1)
[0.455s][trace][gc,heap ] GC(24) Used: 0K, Waste: 0K
[0.455s][info ][gc,heap ] GC(24) Survivor regions: 0->0(1)
[0.455s][trace][gc,heap ] GC(24) Used: 0K, Waste: 0K
[0.455s][info ][gc,heap ] GC(24) Old regions: 2->2
[0.455s][trace][gc,heap ] GC(24) Used: 1931K, Waste: 116K
[0.455s][info ][gc,heap ] GC(24) Humongous regions: 8->8
[0.455s][trace][gc,heap ] GC(24) Used: 8192K, Waste: 0K
[0.455s][debug][gc,heap ] GC(24) Heap after GC invocations=22 (full 13): garbage-first heap total 10240K, used 10123K [0x00000000ff600000, 0x0000000100000000)
[0.455s][debug][gc,heap ] GC(24) region size 1024K, 0 young (0K), 0 survivors (0K)
[0.455s][debug][gc,heap ] GC(24) Metaspace used 6220K, capacity 6395K, committed 6528K, reserved 1056768K
[0.455s][debug][gc,heap ] GC(24) class space used 538K, capacity 606K, committed 640K, reserved 1048576K
[0.455s][info ][gc ] GC(24) Pause Young (Concurrent Start) (G1 Evacuation Pause) 9M->9M(10M) 0.585ms
[0.455s][debug][gc,heap ] GC(25) Heap before GC invocations=22 (full 13): garbage-first heap total 10240K, used 10123K [0x00000000ff600000, 0x0000000100000000)
[0.455s][debug][gc,heap ] GC(25) region size 1024K, 0 young (0K), 0 survivors (0K)
[0.455s][debug][gc,heap ] GC(25) Metaspace used 6220K, capacity 6395K, committed 6528K, reserved 1056768K
[0.455s][debug][gc,heap ] GC(25) class space used 538K, capacity 606K, committed 640K, reserved 1048576K
[0.455s][info ][gc ] GC(26) Concurrent Cycle
[0.457s][debug][gc,ref ] GC(25) Skipped phase3 of Reference Processing due to unavailable references
[0.457s][debug][gc,phases,ref ] GC(25) Reference Processing: 0.0ms
[0.457s][debug][gc,phases,ref ] GC(25) Reconsider SoftReferences: 0.0ms
[0.457s][debug][gc,phases,ref ] GC(25) SoftRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.457s][debug][gc,phases,ref ] GC(25) Notify Soft/WeakReferences: 0.0ms
[0.457s][debug][gc,phases,ref ] GC(25) SoftRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.457s][debug][gc,phases,ref ] GC(25) WeakRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.457s][debug][gc,phases,ref ] GC(25) FinalRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.457s][debug][gc,phases,ref ] GC(25) Total (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.457s][debug][gc,phases,ref ] GC(25) Notify and keep alive finalizable: 0.0ms
[0.457s][debug][gc,phases,ref ] GC(25) FinalRef (ms): skipped
[0.457s][debug][gc,phases,ref ] GC(25) Notify PhantomReferences: 0.0ms
[0.457s][debug][gc,phases,ref ] GC(25) PhantomRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.457s][debug][gc,phases,ref ] GC(25) SoftReference:
[0.457s][debug][gc,phases,ref ] GC(25) Discovered: 6
[0.457s][debug][gc,phases,ref ] GC(25) Cleared: 6
[0.457s][debug][gc,phases,ref ] GC(25) WeakReference:
[0.457s][debug][gc,phases,ref ] GC(25) Discovered: 5
[0.457s][debug][gc,phases,ref ] GC(25) Cleared: 5
[0.457s][debug][gc,phases,ref ] GC(25) FinalReference:
[0.457s][debug][gc,phases,ref ] GC(25) Discovered: 0
[0.457s][debug][gc,phases,ref ] GC(25) Cleared: 0
[0.457s][debug][gc,phases,ref ] GC(25) PhantomReference:
[0.457s][debug][gc,phases,ref ] GC(25) Discovered: 66
[0.457s][debug][gc,phases,ref ] GC(25) Cleared: 66
[0.460s][info ][gc,heap ] GC(25) Eden regions: 0->0(1)
[0.460s][trace][gc,heap ] GC(25) Used: 0K, Waste: 0K
[0.460s][info ][gc,heap ] GC(25) Survivor regions: 0->0(1)
[0.460s][trace][gc,heap ] GC(25) Used: 0K, Waste: 0K
[0.460s][info ][gc,heap ] GC(25) Old regions: 2->2
[0.460s][trace][gc,heap ] GC(25) Used: 1931K, Waste: 116K
[0.460s][info ][gc,heap ] GC(25) Humongous regions: 8->6
[0.460s][trace][gc,heap ] GC(25) Used: 6144K, Waste: 0K
[0.460s][debug][gc,heap ] GC(25) Heap after GC invocations=23 (full 14): garbage-first heap total 10240K, used 8075K [0x00000000ff600000, 0x0000000100000000)
[0.460s][debug][gc,heap ] GC(25) region size 1024K, 0 young (0K), 0 survivors (0K)
[0.460s][debug][gc,heap ] GC(25) Metaspace used 6220K, capacity 6395K, committed 6528K, reserved 1056768K
[0.460s][debug][gc,heap ] GC(25) class space used 538K, capacity 606K, committed 640K, reserved 1048576K
[0.460s][info ][gc ] GC(25) Pause Full (G1 Evacuation Pause) 9M->7M(10M) 5.032ms
[0.461s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28c8
执行了finalize方法
执行了finalize方法
执行了finalize方法
[0.461s][info ][gc ] GC(26) Concurrent Cycle 5.981ms
[0.462s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28d0
[0.462s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28d8
[0.462s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f44b8
[0.462s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b710
[0.462s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b718
[0.462s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f29c0
[0.462s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b720
[0.462s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b728
[0.462s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28e0
[0.462s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28e8
[0.463s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f29c8
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b730
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b740
[0.463s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28f0
[0.463s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f28f8
[0.463s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f2900
[0.463s][info ][oopstorage,ref] StringTable weak: allocated 0x00000187729f2908
[0.463s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f29d0
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b748
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b750
[0.463s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f29d8
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b758
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b760
[0.463s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b768
[0.463s][info ][oopstorage,ref] JNI Global: released 0x000001877265b670
[0.464s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b670
[0.464s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b770
[0.464s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f29e0
[0.464s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b778
[0.464s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b780
[0.464s][info ][oopstorage,ref] JNI Weak: allocated 0x00000187729f29e8
[0.464s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b788
[0.464s][info ][oopstorage,ref] JNI Global: allocated 0x000001877265b790
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at com.gc.demo.finalize.FinalizeGcDemo.main(FinalizeGcDemo.java:26)
日志最后抛出堆内存溢出（java.lang.OutOfMemoryError: Java heap space）异常。
通过MAT工具分析刚才程序运行的dump文件
上面的分析报告，明确指出了Finalizer类已经占用堆内存的51.88%以上，且主要因为GcDemo类下finalize()方法。
打印Reference类的JVM参数设置
- JDK1.9之前 -XX:+PrintReferenceGC
- JDK1.9（含1.9）以后 -Xlog:ref*=debug
日志打印
[0.323s][info ][gc ] GC(3) Pause Full (G1 Humongous Allocation) 8M->7M(10M) 5.099ms
[0.324s][debug][gc,ref ] GC(4) Skipped phase3 of Reference Processing due to unavailable references
[0.324s][debug][gc,phases,ref ] GC(4) Reference Processing: 0.0ms
[0.324s][debug][gc,phases,ref ] GC(4) Reconsider SoftReferences: 0.0ms
[0.324s][debug][gc,phases,ref ] GC(4) SoftRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.324s][debug][gc,phases,ref ] GC(4) Notify Soft/WeakReferences: 0.0ms
[0.324s][debug][gc,phases,ref ] GC(4) SoftRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.324s][debug][gc,phases,ref ] GC(4) WeakRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.324s][debug][gc,phases,ref ] GC(4) FinalRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.324s][debug][gc,phases,ref ] GC(4) Total (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.324s][debug][gc,phases,ref ] GC(4) Notify and keep alive finalizable: 0.0ms
[0.324s][debug][gc,phases,ref ] GC(4) FinalRef (ms): skipped
[0.324s][debug][gc,phases,ref ] GC(4) Notify PhantomReferences: 0.0ms
[0.324s][debug][gc,phases,ref ] GC(4) PhantomRef (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 1
[0.324s][debug][gc,phases,ref ] GC(4) SoftReference:
[0.324s][debug][gc,phases,ref ] GC(4) Discovered: 34
[0.324s][debug][gc,phases,ref ] GC(4) Cleared: 5
[0.324s][debug][gc,phases,ref ] GC(4) WeakReference:
[0.324s][debug][gc,phases,ref ] GC(4) Discovered: 5
[0.324s][debug][gc,phases,ref ] GC(4) Cleared: 5
[0.324s][debug][gc,phases,ref ] GC(4) FinalReference:
[0.324s][debug][gc,phases,ref ] GC(4) Discovered: 0
[0.324s][debug][gc,phases,ref ] GC(4) Cleared: 0
[0.324s][debug][gc,phases,ref ] GC(4) PhantomReference:
[0.324s][debug][gc,phases,ref ] GC(4) Discovered: 62
[0.324s][debug][gc,phases,ref ] GC(4) Cleared: 62
[0.324s][info ][oopstorage,ref] VM Weak Oop Handles: released 0x0000018bb1d48748
[0.324s][info ][oopstorage,ref] VM Weak Oop Handles: released 0x0000018bb1d487b0
[0.324s][info ][oopstorage,ref] VM Weak Oop Handles: released 0x0000018bb1d48660
总结
1. finalize占用内存搞这和GC的机制有关
实现了object的finalize（）的类在创建时会新建一个FinalizerReference，这个对象是强引用类型，封装了override finalize（）的对象，下面直接叫原对象。
原对象没有被其他对象引用时（FinalizeReference除外），执行GC不会马上被清除掉，而是放入一个静态链表中（ReferenceQueue），有一个守护线程专门去维护这个链表，如何维护呢？
就是轮到该线程执行时就弹出里面的对象，执行它们的finalize（），对应的FinalizerReference对象在下次执行GC时就会被清理掉。
一个堆的FinalizerReference会组成一条双向链表，垃圾回收器应该会持有链表头（链表头在FinalizerReference中为一个静态成员）。
2. 为什么会泄漏
直接原因就是守护线程优先级比较低，运行的时间比较少。
如果较短时间内创建较多的原对象，就会因为守护线程来不及弹出原对象而使FinalizerReference和原对象都得不到回收。
无论怎样调用GC都没有用的，因为只要原对象没有被守护线程弹出执行其finalize（）方法，FinalizerReference对象就不会被GC回收。
3. 知道这些机制后,应该注意以下几点
（1）紧缺资源不要依赖finalize()来释放。
（2）尽量不要重载finalize()。
（3）如果必须重载finalize()，一定要记得调用super.finalize，也建议把类实现成单例模式（减少FinalizerReference占用）。
以上为个人经验，希望能给大家一个参考，也希望大家多多支持脚本之家。