Java中String的intern()方法詳細(xì)說(shuō)明

更新時(shí)間：2023年11月17日 09:00:12 作者：top啦它

這篇文章主要介紹了Java中String的intern()方法詳細(xì)說(shuō)明,String::intern()是一個(gè)本地方法,他的作用就是如果字符串常量池中已經(jīng)包含了一個(gè)等于此String對(duì)象的字符串,則返回代表池中的這個(gè)字符串額String對(duì)象的引用,需要的朋友可以參考下

String的intern()方法

先看第一個(gè)例子

public class HelloWorld {
    public static void main(String[] args) {
        String s1 = new String("hello") + new String("world");
        s1.intern();
        String s2 = "helloworld";
        System.out.println(s1==s2);
    }
}

輸出為：true

再看第二個(gè)例子

public class HelloWorld {
    public static void main(String[] args) {
        String s1 = new String("hello") + new String("world");
        String s2 = "helloworld";
        s1.intern();
        System.out.println(s1==s2);
    }
}

輸出為：false

明明只有一行代碼的位置發(fā)生了改變，為什么會(huì)輸出截然不同的結(jié)果呢？

下面開始分析。

jdk1.7后的intern方法在調(diào)用后，存在兩種情況。

一、在調(diào)用后發(fā)現(xiàn)StringTable中沒(méi)有所對(duì)應(yīng)的字符串

String s1 = new String("hello") + new String("world");

這句代碼會(huì)在堆中new一個(gè)String對(duì)象（其實(shí)并沒(méi)有這么簡(jiǎn)單，后面講）

s1.intern();

s1調(diào)用了intern方法，然后發(fā)現(xiàn)在StringTable中并沒(méi)有所對(duì)應(yīng)的字符串，那么jvm就會(huì)在StringTable中放入一個(gè)地址，這個(gè)地址指向s1所new的String對(duì)象。

String s2 = "helloworld";

jvm發(fā)現(xiàn)在StringTable中已經(jīng)存在指向"helloworld"的地址了，就會(huì)把StringTable中的地址給s2。此時(shí)s1和s2都指向這個(gè)地址。所以是同一個(gè)對(duì)象。于是返回了true

二、在調(diào)用后發(fā)現(xiàn)StringTable中存在所對(duì)應(yīng)的字符串

String s1 = new String("hello") + new String("world");

這句代碼會(huì)在堆中new一個(gè)String對(duì)象（其實(shí)并沒(méi)有這么簡(jiǎn)單，后面講）

String s2 = "helloworld";

jvm在StringTable中存儲(chǔ)這一個(gè)"helloworld"。

此時(shí)s1和s2是兩個(gè)不同的對(duì)象。s1指向堆中，s2指向StringTable。

s1.intern();

jvm發(fā)現(xiàn)在StringTable中已經(jīng)存在"helloworld"了，那么什么都不會(huì)做，僅僅是返回這一個(gè)字符串。所以結(jié)果為false

下面講一下String s1 = new String("hello") + new String("world"); 底層做了什么。

public class HelloWorld {
    public static void main(String[] args) {
        String s1 = new String("hello") + new String("world");
    }
}

先運(yùn)行一遍，類加載器加載后，在target中會(huì)存在HelloWorld.class

public class HelloWorld {
    public HelloWorld() {
    }

    public static void main(String[] args) {
        String s1 = new String("hello") + new String("world");
    }
}

運(yùn)行指令javap -v target/classes/aa/HelloWorld.class進(jìn)行反編譯。反編譯后的詳細(xì)信息如下：

Classfile /C:/Users/top/Downloads/untitled5/target/classes/aa/HelloWorld.class
  Last modified 2022年4月14日; size 677 bytes
  SHA-256 checksum bf071098fdd9e51e927c011bb021cab16ac1227bd18a6435441f30e30b400f70
  Compiled from "HelloWorld.java"
public class aa.HelloWorld
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #10                         // aa/HelloWorld
  super_class: #11                        // java/lang/Object
  interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
   #1 = Methodref          #11.#27        // java/lang/Object."<init>":()V
   #2 = Class              #28            // java/lang/StringBuilder
   #3 = Methodref          #2.#27         // java/lang/StringBuilder."<init>":()V
   #4 = Class              #29            // java/lang/String
   #5 = String             #30            // hello
   #6 = Methodref          #4.#31         // java/lang/String."<init>":(Ljava/lang/String;)V
   #7 = Methodref          #2.#32         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = String             #33            // world
   #9 = Methodref          #2.#34         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #10 = Class              #35            // aa/HelloWorld
  #11 = Class              #36            // java/lang/Object
  #12 = Utf8               <init>
  #13 = Utf8               ()V
  #14 = Utf8               Code
  #15 = Utf8               LineNumberTable
  #16 = Utf8               LocalVariableTable
  #17 = Utf8               this
  #18 = Utf8               Laa/HelloWorld;
  #19 = Utf8               main
  #20 = Utf8               ([Ljava/lang/String;)V
  #21 = Utf8               args
  #22 = Utf8               [Ljava/lang/String;
  #23 = Utf8               s1
  #24 = Utf8               Ljava/lang/String;
  #25 = Utf8               SourceFile
  #26 = Utf8               HelloWorld.java
  #27 = NameAndType        #12:#13        // "<init>":()V
  #28 = Utf8               java/lang/StringBuilder
  #29 = Utf8               java/lang/String
  #30 = Utf8               hello
  #31 = NameAndType        #12:#37        // "<init>":(Ljava/lang/String;)V
  #32 = NameAndType        #38:#39        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #33 = Utf8               world
  #34 = NameAndType        #40:#41        // toString:()Ljava/lang/String;
  #35 = Utf8               aa/HelloWorld
  #36 = Utf8               java/lang/Object
  #37 = Utf8               (Ljava/lang/String;)V
  #38 = Utf8               append
  #39 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #40 = Utf8               toString
  #41 = Utf8               ()Ljava/lang/String;
{
  public aa.HelloWorld();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Laa/HelloWorld;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=2, args_size=1
         0: new           #2                  // class java/lang/StringBuilder
         3: dup
         4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
         7: new           #4                  // class java/lang/String
        10: dup
        11: ldc           #5                  // String hello
        13: invokespecial #6                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
        16: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        19: new           #4                  // class java/lang/String
        22: dup
        23: ldc           #8                  // String world
        25: invokespecial #6                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
        28: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        31: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        34: astore_1
        35: return
      LineNumberTable:
        line 5: 0
        line 6: 35
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      36     0  args   [Ljava/lang/String;
           35       1     1    s1   Ljava/lang/String;
}
SourceFile: "HelloWorld.java"