Java內部類持有外部類的引用詳細分析與解決方案

在Java中內部類的定義與使用通常爲成員內部類與匿名內部類，他們的對象都會隱式持有外部類對象的引用，影響外部類對象的回收。

GC只會回收沒有被引用或者根集不可到達的對象（取決於GC算法），內部類在生命週期內始終持有外部類的對象的引用，形成外部類的對象始終不知足GC的回收條件，反映在內存上就是內存泄露。（如，Android中Activity的內存泄露）

解決方案爲

1.將內部類定義爲static

2.用static的變量引用匿名內部類的實例

測試代碼

class Outer {
    class Inner {
        public String publicString = "Inner.publicString";
    }

    Other anonymousOther = new Other() {
        public String publicString = "Anonymous Other.publicString";
    };
    public Other getAnonymousOther() {
        return anonymousOther;
    }

    Other Other = new Other();
    public Other getOther() {
        return Other;
    }
}

class Other {
    public String publicString = "Other.publicString";
}

調用代碼

public static void main(String args[]) {
        printField(new Outer().new Inner());
        System.out.println("\t");
        printField(new Outer().getAnonymousOther());
        System.out.println("\t");
        printField(new Outer().getOther());
    }

 

測試結果

Class: at.miao.Outer$Inner
變量: publicString 值爲 Inner.publicString
變量: this$0 值爲 at.miao.Outer@456c5f50

Class: at.miao.Outer$1
變量: publicString 值爲 Anonymous Other.publicString
變量: this$0 值爲 at.miao.Outer@10e80317
Class: at.miao.Other
變量: publicString 值爲 Other.publicString

能夠看到內部類與匿名內部類的實例都有一個外部類類型的名爲this$0的變量指向了外部類的對象。

加上static以後，代碼爲

class Outer {
    static class Inner {
        public String publicString = "Inner.publicString";
    }

    static Other anonymousOther = new Other() {
        public String publicString = "Anonymous Other.publicString";
    };

    public Other getAnonymousOther() {
        return anonymousOther;
    }

    Other Other = new Other();
    public Other getOther() {
        return Other;
    }
}

class Other {
    public String publicString = "Other.publicString";
}

調用代碼

public static void main(String args[]) {
        printField(new Outer.Inner());
        System.out.println("\t");
        printField(new Outer().getAnonymousOther());
        System.out.println("\t");
        printField(new Outer().getOther());
    }

 

測試結果

Class: at.miao.Outer$Inner
變量: publicString 值爲 Inner.publicString

Class: at.miao.Outer$1
變量: publicString 值爲 Anonymous Other.publicString

Class: at.miao.Other
變量: publicString 值爲 Other.publicString

能夠看到靜態內部類實例、static引用的匿名內部類的實例未引用外部類的實例。