java – String常量池和实习生

我最近几天试图理解String常量池和inter的概念,在阅读了很多文章之后我理解了它的一些部分,但仍然对以下几点感到困惑： –

1.String a =“abc”这将在String Constant Pool中创建一个对象
但是以下代码行是否在字符串常量池中创建对象“xyz”？
字符串b =(“xyz”).toLowerCase()

2.

String c = "qwe"   
String d = c.substring(1)    
d.intern()   
String e = "we"

如果在类加载期间将文字“we”添加到String consant池中,如果是这样,为什么d == e结果为true,即使d未指向String Constant池也是如此

解决方法

令人惊讶的是,我们可以在常量池之前影响字符串池;正如下面的代码片段所示.

要理解为什么两个代码片段具有不同的行为,重要的是要清楚

> the constant pool is not the same as the string pool.也就是说,常量池是存储在磁盘上的类文件的一部分,字符串池是用字符串填充的运行时缓存.
>并且根据Java语言规范jls-3.10.5,引用字符串文字并不直接引用常量池;当且仅当字符串池中没有值时,字符文字才会从常量池填充字符串池.

也就是说,从源文件到运行时的String对象的生命周期如下：

>在编译时由编译器放入常量池并存储在生成的类文件中(每个类文件有一个常量池)
> JVM在类加载时加载常量池
>从常量池创建的字符串在运行时被添加到字符串池中,因为调用实习生(如果已经存在等效字符串,如果已经存在字符串,那么将使用字符串池中的字符串)JVM Spec 5.1 – The Run-Time Constant Pool.
> intern可以通过手动调用intern()显式发生,也可以通过引用字符串文字(如“abc”jls-3.10.5)隐式发生.

以下两个代码片段之间的行为差异是由于在通过字符串文字发生对内部的隐式调用之前显式调用intern()引起的.

为清楚起见,这里是对这个答案的评论中讨论的两种行为的贯彻：

String c = "qwe";   // string literal qwe goes into runtime cache
    String d = c.substring(1); // runtime string "we" is created
    d.intern();         // intern "we"; it has not been seen 
                        // yet so this version goes into the cache
    String e = "we";    // now we see the string literal,but
                        // a value is already in the cache and so 
                        // the same instance as d is returned 
                        // (see ref below)

    System.out.println( e == d );  // returns true

以下是在使用字符串文字后我们实习的情况：

String c = "qwe";   // string literal qwe goes into runtime cache
    String d = c.substring(1); // runtime string "we" is created
    String e = "we";    // now we see the string literal,this time
                        // a value is NOT already in the cache and so 
                        // the string literal creates an object and
                        // places it into the cache
    d.intern();         // has no effect - a value already exists
                        // in the cache,and so it will return e

    System.out.println( e == d );  // returns false
    System.out.println( e == d.intern() );  // returns true
    System.out.println( e == d );  // still returns false

下面是JLS的关键部分,声明实际上是为字符串文字调用实习生.

Moreover,a string literal always refers to the same instance of class String. This is because string literals – or,more generally,strings that are the values of constant expressions (§15.28) – are “interned” so as to share unique instances,using the method String.intern.

JVM规范涵盖了从类文件加载的常量池的运行时表示的详细信息,并且它与实习生交互.

If the method String.intern has previously been called on an instance of class String containing a sequence of Unicode code points identical to that given by the CONSTANT_String_info structure,then the result of string literal derivation is a reference to that same instance of class String. .

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