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關(guān)于JVM垃圾回收的java.lang.ref.Finalizer問題

更新時間：2024年05月16日 09:05:35 作者：影??

這篇文章主要介紹了關(guān)于JVM垃圾回收的java.lang.ref.Finalizer問題,具有很好的參考價值,希望對大家有所幫助,如有錯誤或未考慮完全的地方,望不吝賜教

JVM垃圾回收的java.lang.ref.Finalizer

基礎(chǔ)知識

Shallow Size

對象自身占用的內(nèi)存大小，不包括它引用的對象

Retained Size

Retained Size=當(dāng)前對象大小+當(dāng)前對象可直接或間接引用到的對象的大小總和

可達性分析算法：

基本思路是通過一系列成為GC ROOTS 的對象作為起點，當(dāng)一個對象到 GC ROOTS 沒有任何相連，證明此對象是不可達，即被判斷為可回收的對象。

之后的過程是：

被標(biāo)記不可達的對象以后，進行第一次標(biāo)記，和第一次篩選，條件是該對象有沒有必要執(zhí)行finalize方法

沒有必要執(zhí)行finalize方法的情況是

1.finalize已經(jīng)執(zhí)行過（finalize 方法只會被執(zhí)行一次）

2.該對象沒有重寫finalize方法

如果要執(zhí)行finalize方法，該對象進入一個F-Queue隊列，稍后有一個優(yōu)先級為8的 finalizer線程來執(zhí)行（注意：如果一個對象在 finalize 方法中運行緩慢，將會導(dǎo)致隊列后的其他對象永遠等待，嚴(yán)重時將會導(dǎo)致系統(tǒng)崩潰）

GC對隊列中進行第二次標(biāo)記，如果在執(zhí)行finalize方法的時候?qū)⒆约汉虶C ROOTS關(guān)聯(lián)上，該對象即可逃離回收，否則，被回收掉

重要

對象是在已經(jīng)被GC識別為是垃圾后才丟到Queue中的，在queue中依然占用內(nèi)存

引用對象的類型

引用對象的類型第一篇已說過，分為以下類型

強引用>軟引用>弱引用>虛引用，

涉及到的相關(guān)類：

軟引用:softReference
弱引用:weekRefernce
虛引用:pathomReference

但還有一個特殊的引用：FinalReference類，該類也是JVM調(diào)優(yōu)的常見的調(diào)優(yōu)場景之一。

要了解FinalReference,先了解final()

final,finally和finalize()的區(qū)別？

前兩者我相信大多數(shù)人都能答出，但finalize()雖然也有人答出，但可能只是硬搬面試題答案。

1.final：修飾詞，作用于成員變量，方法和類。

成員變量，基本數(shù)據(jù)類型表示該成員變量不可變更數(shù)值，對象表示該對象引用地址不可改變。
方法，表示該方法不可重載
類，表示該類不可被繼承

2.finally：是異常處理機制的一部分，表示總是執(zhí)行這段代碼，先執(zhí)行異常，后執(zhí)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("執(zhí)行異常");
        } finally {
            log.info("執(zhí)行finally片段");
        }
    }
}

// 打印結(jié)果：

// 10:36:09.777 [main] INFO com.gc.demo.finalize.finlly.FinallyDemo - 執(zhí)行異常

// 10:36:09.780 [main] INFO com.gc.demo.finalize.finlly.FinallyDemo - 執(zhí)行finally片段

3.finalize():是object的一個方法。簡簡單單的一句話，我想大多數(shù)人懂得，但其實我們應(yīng)該繼續(xù)深入。

Finalize()

Object源碼

public class Object {
    private static native void registerNatives();
    static {
        registerNatives();
    }
    .......
    @Deprecated(since="9")
    protected void finalize() throws Throwable { }
}

上述代碼可以看出：finalize()已被棄用，不建議使用

為什么finalize()已被棄用，不建議使用？

原因：容易導(dǎo)致堆內(nèi)存溢出。

為什么不直接刪除呢？

1. 兼容老舊項目

2. 是GC回收對象前，對該對象生前（被回收前）必須執(zhí)行的邏輯業(yè)務(wù)，保證程序正常運行。例如：FileInputStream，源碼如下

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類的close()方法大家都不陌生，用于關(guān)閉流輸入，但他被寫到finalize()方法內(nèi)。

原理：

JVM啟動時，檢測所有對象，是否實現(xiàn)了finalize()方法，如果實現(xiàn)了，就會該對象標(biāo)記為finalizer類，且交由finalizerThread線程管理，當(dāng)GC回收時，finalizerThread線程就會處理finalizer類，檢測是否有執(zhí)行finalize()方法，如果有，則可以把finalizer從線程中去除，對應(yīng)的對象就可以被回收；如果沒有，則一直交由finalizerThread線程管理。

詳細流程

JVM啟動時，檢測對象是否實現(xiàn)了finalize()方法，檢測方法：判斷has_finalizer_flag和registerfinalizerrsaltInit字段

A對象實現(xiàn)了finalize()方法，就會把A對象的finalize()函數(shù)注冊到finalizerthread線程和referencequeue隊列中，注冊完畢后，且稱該函數(shù)注冊的B對象稱為Finalizer類，指向原有的A對象。

GC回收垃圾時，啟動FinalizerThread線程，檢測Finalizer類對應(yīng)的A對象的finalize()方法是否已執(zhí)行，已調(diào)用，則可以把Finalizer類去除，回收Finalizer類，從而回收A對象；如果沒有調(diào)用，則繼續(xù)交由FinalizerThread線程管理，不回收。

從上面原因引申出：為什么堆內(nèi)存溢出？

GC回收時，用戶線程、GC回收線程和FinalizerThread線程都在運行，但由于FinalizerThread線程優(yōu)先度比GC回收線程和用戶線程低，所以導(dǎo)致回收速度比對象創(chuàng)建速度慢，最終導(dǎo)致堆內(nèi)存溢出。

示例程序：

JDK11、G1垃圾收集器

JVM設(shè)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("執(zhí)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
執(zhí)行了finalize方法
執(zhí)行了finalize方法
執(zhí)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)

日志最后拋出堆內(nèi)存溢出（java.lang.OutOfMemoryError: Java heap space）異常。

通過MAT工具分析剛才程序運行的dump文件

上面的分析報告，明確指出了Finalizer類已經(jīng)占用堆內(nèi)存的51.88%以上，且主要因為GcDemo類下finalize()方法。

打印Reference類的JVM參數(shù)設(shè)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