详细分析内部类的发生内存泄漏的原因

文章目录

避免内部类中的内存泄漏
步骤1：内部类引用其外部类
步骤2：构造函数获取封闭的类引用
步骤3：声明一种新方法
内存泄漏的解剖

避免内部类中的内存泄漏

使用内部类时要当心垃圾收集
如果您已了解静态类和内部类，则应该熟悉使用Java代码中的嵌套类的基础知识。在这个相关的技巧中，我将带您了解嵌套类的陷阱之一，这是内部类在JVM中导致内存泄漏和内存不足错误的潜力。

之所以会发生这种类型的内存泄漏，是因为内部类必须始终能够访问其外部类-并非总是与JVM的计划一起使用。

从简单的嵌套过程到内存不足错误（并可能关闭JVM）是一个过程。理解它的最好方法是看它的源码。

步骤1：内部类引用其外部类

内部类的任何实例都包含对其外部类的隐式引用。例如，考虑以下EnclosingClass带有嵌套的EnclosedClass非静态成员类的声明：

public class EnclosingClass
{public class EnclosedClass{}
}

为了更好地理解这种联系，我们可以将上面的源代码（javac EnclosingClass.java）编译为EnclosingClass.class和EnclosingClass$EnclosedClass.class，然后检查后者的类文件。

JDK包含一个javap（Java打印）工具，用于反汇编类文件。在命令行上，使用javap运行EnclosingClass$EnclosedClass如下：

javap EnclosingClass$EnclosedClass

您可以看到以下输出，该输出显示了包含以下内容final EnclosingClass this$0字段EnclosingClass：

public class com.github.crab2died.EnclosingClass$EnclosedClass {final com.github.crab2died.EnclosingClass this$0;public com.github.crab2died.EnclosingClass$EnclosedClass(com.github.crab2died.EnclosingClass);
}

步骤2：构造函数获取封闭的类引用

上面的输出显示了带有EnclosingClass参数的构造函数。使用javap -v（verbose）选项执行，您将观察到构造函数将EnclosingClass对象引用保存在this$0字段中：

Classfile /D:/aplus/EnclosingClass$EnclosedClass.classLast modified 2020-3-15; size 440 bytesMD5 checksum 308ea24edb49a4d49669d101fff55d5aCompiled from "EnclosingClass.java"
public class com.github.crab2died.EnclosingClass$EnclosedClassminor version: 0major version: 52flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Fieldref           #3.#13         // com/github/crab2died/EnclosingClass$EnclosedClass.this$0:Lcom/github/crab2died/EnclosingClass;#2 = Methodref          #4.#14         // java/lang/Object."<init>":()V#3 = Class              #16            // com/github/crab2died/EnclosingClass$EnclosedClass#4 = Class              #19            // java/lang/Object#5 = Utf8               this$0#6 = Utf8               Lcom/github/crab2died/EnclosingClass;#7 = Utf8               <init>#8 = Utf8               (Lcom/github/crab2died/EnclosingClass;)V#9 = Utf8               Code#10 = Utf8               LineNumberTable#11 = Utf8               SourceFile#12 = Utf8               EnclosingClass.java#13 = NameAndType        #5:#6          // this$0:Lcom/github/crab2died/EnclosingClass;#14 = NameAndType        #7:#20         // "<init>":()V#15 = Class              #21            // com/github/crab2died/EnclosingClass#16 = Utf8               com/github/crab2died/EnclosingClass$EnclosedClass#17 = Utf8               EnclosedClass#18 = Utf8               InnerClasses#19 = Utf8               java/lang/Object#20 = Utf8               ()V#21 = Utf8               com/github/crab2died/EnclosingClass
{final com.github.crab2died.EnclosingClass this$0;descriptor: Lcom/github/crab2died/EnclosingClass;flags: ACC_FINAL, ACC_SYNTHETICpublic com.github.crab2died.EnclosingClass$EnclosedClass(com.github.crab2died.EnclosingClass);descriptor: (Lcom/github/crab2died/EnclosingClass;)Vflags: ACC_PUBLICCode:stack=2, locals=2, args_size=20: aload_01: aload_12: putfield      #1                  // Field this$0:Lcom/github/crab2died/EnclosingClass;5: aload_06: invokespecial #2                  // Method java/lang/Object."<init>":()V9: returnLineNumberTable:line 12: 0
}
SourceFile: "EnclosingClass.java"
InnerClasses:public #17= #3 of #15; //EnclosedClass=class com/github/crab2died/EnclosingClass$EnclosedClass of class com/github/crab2died/EnclosingClass

步骤3：声明一种新方法

在实例化类中声明了EnclosingClass，然后创建EnclosedClass。

EnclosingClass ec = new EnclosingClass();
ec.new EnclosedClass();

 Last modified 2020-3-15; size 502 bytesMD5 checksum d31832f98dbbf557e995ac447cc55fb2Compiled from "EnclosingClass.java"
public class com.github.crab2died.EnclosingClassminor version: 0major version: 52flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Methodref          #7.#17         // java/lang/Object."<init>":()V#2 = Class              #18            // com/github/crab2died/EnclosingClass#3 = Methodref          #2.#17         // com/github/crab2died/EnclosingClass."<init>":()V#4 = Class              #19            // com/github/crab2died/EnclosingClass$EnclosedClass#5 = Methodref          #7.#20         // java/lang/Object.getClass:()Ljava/lang/Class;#6 = Methodref          #4.#21         // com/github/crab2died/EnclosingClass$EnclosedClass."<init>":(Lcom/github/crab2died/EnclosingClass;)V#7 = Class              #22            // java/lang/Object#8 = Utf8               EnclosedClass#9 = Utf8               InnerClasses#10 = Utf8               <init>#11 = Utf8               ()V#12 = Utf8               Code#13 = Utf8               LineNumberTable#14 = Utf8               test#15 = Utf8               SourceFile#16 = Utf8               EnclosingClass.java#17 = NameAndType        #10:#11        // "<init>":()V#18 = Utf8               com/github/crab2died/EnclosingClass#19 = Utf8               com/github/crab2died/EnclosingClass$EnclosedClass#20 = NameAndType        #23:#24        // getClass:()Ljava/lang/Class;#21 = NameAndType        #10:#25        // "<init>":(Lcom/github/crab2died/EnclosingClass;)V#22 = Utf8               java/lang/Object#23 = Utf8               getClass#24 = Utf8               ()Ljava/lang/Class;#25 = Utf8               (Lcom/github/crab2died/EnclosingClass;)V
{public com.github.crab2died.EnclosingClass();descriptor: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1                  // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 10: 0public void test();descriptor: ()Vflags: ACC_PUBLICCode:stack=4, locals=2, args_size=10: new           #2                  // class com/github/crab2died/EnclosingClass3: dup4: invokespecial #3                  // Method "<init>":()V7: astore_18: new           #4                  // class com/github/crab2died/EnclosingClass$EnclosedClass11: dup12: aload_113: dup14: invokevirtual #5                  // Method java/lang/Object.getClass:()Ljava/lang/Class;17: pop18: invokespecial #6                  // Method com/github/crab2died/EnclosingClass$EnclosedClass."<init>":(Lcom/github/crab2died/EnclosingClass;)V21: pop22: returnLineNumberTable:line 17: 0line 18: 8line 19: 22
}
SourceFile: "EnclosingClass.java"
InnerClasses:public #8= #4 of #2; //EnclosedClass=class com/github/crab2died/EnclosingClass$EnclosedClass of class com/github/crab2died/EnclosingClass

内存泄漏的解剖

在以上示例中，我们已将封闭类的引用存储在封闭类的制造变量中。这可能导致内存泄漏，其中的封闭类引用了无法垃圾回收的大型对象图。根据应用程序代码，可能会耗尽内存并收到内存不足错误，从而导致JVM终止。下面的例子演示这种情况。

MemoryLeak.java

import java.util.ArrayList;class EnclosingClass
{private int[] data;public EnclosingClass(int size){data = new int[size];}class EnclosedClass{}EnclosedClass getEnclosedClassObject(){return new EnclosedClass();}
}public class MemoryLeak
{public static void main(String[] args){ArrayList al = new ArrayList<>();int counter = 0;while (true){al.add(new EnclosingClass(100000).getEnclosedClassObject());System.out.println(counter++);}}
}

该EnclosingClass声明一个私有data引用整数数组领域。数组的大小传递给此类的构造函数，并实例化该数组。

的EnclosingClass还声明EnclosedClass，一个嵌套非静态成员的类，和一种方法，其实例化EnclosedClass，返回此实例。

MemoryLeak的main()方法首先创建一个java.util.ArrayList存储EnclosingClass.EnclosedClass对象。暂时不使用包和泛型以及将包和泛型ArrayList（将对象存储在动态数组中）的使用-重要的一点是观察内存泄漏是如何发生的。

将计数器初始化为0后，main()进入无限while循环，该循环重复实例化EnclosedClass并将其添加到数组列表中。然后打印（或递增）计数器。在实例化封闭的类之前，EnclosingClass必须实例化该实例，并将100000其作为数组大小传递。

每个存储的EnclosedClass对象维护对其封闭对象的引用，该对象引用100,000个32位整数（或400,000字节）的数组。在对内部对象进行垃圾收集之前，无法对外部对象进行垃圾收集。最终，该应用程序将耗尽内存。

我观察到输出的以下后缀-请注意，您可能会观察到不同的最终计数器值：


7639
7640
7641
7642
7643
7644
7645
Exception in thread "main" java.lang.OutOfMemoryError: Java heap spaceat EnclosingClass.<init>(MemoryLeak.java:9)at MemoryLeak.main(MemoryLeak.java:30)