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JVM:运行时方法区之常量池

Java JVM 大约 7622 字

Java 代码

public class StringTableDemo {

    public static void main(String[] args) {
        String s1 = "a";
        String s2 = "b";
        String s3 = "ab";
        String s4 = s1 + s2;
        String s5 = "a" + "b";
        System.out.println(s3 == s4);
        System.out.println(s3.equals(s4));
        System.out.println(s3 == s5);
    }

}

Class 字节码

javap -v .\StringTableDemo.class

字节码文件(注释了输出语句后编译的结果)

Classfile /D:/jvm/StringTableDemo.class
  Last modified Dec 17, 2021; size 712 bytes
  MD5 checksum 7c317c34bf6b2d10ebe7897c17224805
  Compiled from "StringTableDemo.java"
public class jvm.StringTableDemo
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #9                          // jvm/StringTableDemo
  super_class: #10                        // java/lang/Object
  interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
   #1 = Methodref          #10.#30        // java/lang/Object."<init>":()V
   #2 = String             #31            // a
   #3 = String             #32            // b
   #4 = String             #33            // ab
   #5 = Class              #34            // java/lang/StringBuilder
   #6 = Methodref          #5.#30         // java/lang/StringBuilder."<init>":()V
   #7 = Methodref          #5.#35         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = Methodref          #5.#36         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   #9 = Class              #37            // jvm/StringTableDemo
  #10 = Class              #38            // java/lang/Object
  #11 = Utf8               <init>
  #12 = Utf8               ()V
  #13 = Utf8               Code
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               Ljvm/StringTableDemo;
  #18 = Utf8               main
  #19 = Utf8               ([Ljava/lang/String;)V
  #20 = Utf8               args
  #21 = Utf8               [Ljava/lang/String;
  #22 = Utf8               s1
  #23 = Utf8               Ljava/lang/String;
  #24 = Utf8               s2
  #25 = Utf8               s3
  #26 = Utf8               s4
  #27 = Utf8               s5
  #28 = Utf8               SourceFile
  #29 = Utf8               StringTableDemo.java
  #30 = NameAndType        #11:#12        // "<init>":()V
  #31 = Utf8               a
  #32 = Utf8               b
  #33 = Utf8               ab
  #34 = Utf8               java/lang/StringBuilder
  #35 = NameAndType        #39:#40        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #36 = NameAndType        #41:#42        // toString:()Ljava/lang/String;
  #37 = Utf8               jvm/StringTableDemo
  #38 = Utf8               java/lang/Object
  #39 = Utf8               append
  #40 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #41 = Utf8               toString
  #42 = Utf8               ()Ljava/lang/String;
{
  public jvm.StringTableDemo();
    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   Ljvm/StringTableDemo;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=6, args_size=1
         0: ldc           #2                  // String a
         2: astore_1
         3: ldc           #3                  // String b
         5: astore_2
         6: ldc           #4                  // String ab
         8: astore_3
         9: new           #5                  // class java/lang/StringBuilder
        12: dup
        13: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V
        16: aload_1
        17: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        20: aload_2
        21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        24: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        27: astore        4
        29: ldc           #4                  // String ab
        31: astore        5
        33: return
      LineNumberTable:
        line 31: 0
        line 32: 3
        line 33: 6
        line 34: 9
        line 35: 29
        line 40: 33
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      34     0  args   [Ljava/lang/String;
            3      31     1    s1   Ljava/lang/String;
            6      28     2    s2   Ljava/lang/String;
            9      25     3    s3   Ljava/lang/String;
           29       5     4    s4   Ljava/lang/String;
           33       1     5    s5   Ljava/lang/String;
}
SourceFile: "StringTableDemo.java"

常量池

常量池在每个字节码文件的Constant pool中。

虚拟机指令根据这张常量表找到要执行的类名、方法名、参数类型、字面量信息等。

Constant pool:
   #1 = Methodref          #10.#30        // java/lang/Object."<init>":()V
   #2 = String             #31            // a
   #3 = String             #32            // b
   #4 = String             #33            // ab
   #5 = Class              #34            // java/lang/StringBuilder
   #6 = Methodref          #5.#30         // java/lang/StringBuilder."<init>":()V
   #7 = Methodref          #5.#35         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = Methodref          #5.#36         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   #9 = Class              #37            // jvm/StringTableDemo
  #10 = Class              #38            // java/lang/Object
...

运行时常量池

当类被加载,它的常量池信息就会被放入到运行时常量池,并把里面的符号地址变为真实内存地址。

字符串常量池

常量池中的信息,都会被加载到运行时常量池中,这时abab都是常量池中的符号,还没有变为Java字符串对象。

  • ldc #2会把a符号变为a字符串对象。
  • ldc #3会把b符号变为b字符串对象。
  • ldc #4会把ab符号变为ab字符串对象。

StringTable ["a", "b", "ab"]hashtable结构,不能扩容。

StringBuilder最后toStringnewString用的是char数组,所以不会在字符串常量池中加入字符串。

public String(char value[], int offset, int count)

    public static void main(String[] args) {
        String s1 = "a";
        String s2 = "b";
        String s3 = "ab";
        String s4 = s1 + s2; // new StringBuffer().append("a").append("b").toString() // new String("ab")
        String s5 = "a" + "b"; // javac 在编译期间的优化,结果已经在编译期已经确定为 ab

//        System.out.println(s3 == s4); // false s3 指向字符串常量池中的 ab,s4 指向堆中的 new String("ab") 对象
//        System.out.println(s3.equals(s4)); // true
//        System.out.println(s3 == s5); // true
    }

StringTable特性

  • 常量池中的字符串仅仅是符号,第一次用到时才变为对象(延迟加载)。
  • 利用字符串常量池的机制,来避免重复创建字符串对象。
  • 字符串变量拼接的原理是StringBuilderJava8)。
  • 字符串常量拼接的原理是编译期优化。
  • 可以使用intern方法,主动将字符串常量池还没有的字符串对象放入字符串常量池中。

intern

s1 == s2falsexy先放入字符串常量池中,s1执行常量池中的xys2指向堆内存中的StringBuilder生成的String,再调用intern方法发现字符串常量池中已经有xy了,不再放入,引用也不发生改变继续指向堆内存中。所以不相等。

public static void main(String[] args) {
    String s1 = "xy";
    String s2 = new String("x") + new String("y");
    s2.intern();
    System.out.println(s1 == s2);
}

s1 == s2trues2指向堆内存中的StringBuilder生成的String,再调用intern方法发现字符串常量池中没有xy,放入字符串常量池中,且将引用指向字符串常量池中的xy;而s1指向xy发现字符串常量池已经有xy了,直接指向字符串常量池中的xy,所以s1s2都指向字符串常量池中的xy,所以相等。

public static void main(String[] args) {
    String s2 = new String("x") + new String("y");
    s2.intern();
    String s1 = "xy";
    System.out.println(s1 == s2);
}
阅读 120 · 发布于 2022-05-28

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