StringTable

首先看这样一个面试题

// StringTable [ "a", "b" ,"ab" ]  hashtable 结构，不能扩容
public class Demo1_22 {
    // 常量池中的信息，都会被加载到运行时常量池中， 这时 a b ab 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

    public static void main(String[] args) {
        String s1 = "a"; // 懒惰的
        String s2 = "b";
        String s3 = "ab";
        String s4 = s1 + s2; // new StringBuilder().append("a").append("b").toString()  new String("ab")
        String s5 = "a" + "b";  // javac 在编译期间的优化，结果已经在编译期确定为ab
        System.out.println(s3 == s4); // false
        System.out.println(s3 == s5); // true
    }
}

我们从字节码的角度来分析结果

首先反编译该类代字节码,输入命令：

javap -v Demo1_22.class // -v显示详细信息

得到结果：

Classfile /D:/work_folder/java_studying/idea_workspace/jvm/out/production/jvm/cn/itcast/jvm/t1/stringtable/Demo1_22.class
  Last modified 2019-12-8; size 1039 bytes
  MD5 checksum 9efed67948c7dacf9ddb854ca18f8c2b
  Compiled from "Demo1_22.java"
public class cn.itcast.jvm.t1.stringtable.Demo1_22
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #12.#36        // java/lang/Object."<init>":()V
   #2 = String             #37            // a
   #3 = String             #38            // b
   #4 = String             #39            // ab
   #5 = Class              #40            // java/lang/StringBuilder
   #6 = Methodref          #5.#36         // java/lang/StringBuilder."<init>":()V
   #7 = Methodref          #5.#41         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = Methodref          #5.#42         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   #9 = Fieldref           #43.#44        // java/lang/System.out:Ljava/io/PrintStream;
  #10 = Methodref          #45.#46        // java/io/PrintStream.println:(Z)V
  #11 = Class              #47            // cn/itcast/jvm/t1/stringtable/Demo1_22
  #12 = Class              #48            // java/lang/Object
  #13 = Utf8               <init>
  #14 = Utf8               ()V
  #15 = Utf8               Code
  #16 = Utf8               LineNumberTable
  #17 = Utf8               LocalVariableTable
  #18 = Utf8               this
  #19 = Utf8               Lcn/itcast/jvm/t1/stringtable/Demo1_22;
  #20 = Utf8               main
  #21 = Utf8               ([Ljava/lang/String;)V
  #22 = Utf8               args
  #23 = Utf8               [Ljava/lang/String;
  #24 = Utf8               s1
  #25 = Utf8               Ljava/lang/String;
  #26 = Utf8               s2
  #27 = Utf8               s3
  #28 = Utf8               s4
  #29 = Utf8               s5
  #30 = Utf8               StackMapTable
  #31 = Class              #23            // "[Ljava/lang/String;"
  #32 = Class              #49            // java/lang/String
  #33 = Class              #50            // java/io/PrintStream
  #34 = Utf8               SourceFile
  #35 = Utf8               Demo1_22.java
  #36 = NameAndType        #13:#14        // "<init>":()V
  #37 = Utf8               a
  #38 = Utf8               b
  #39 = Utf8               ab
  #40 = Utf8               java/lang/StringBuilder
  #41 = NameAndType        #51:#52        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #42 = NameAndType        #53:#54        // toString:()Ljava/lang/String;
  #43 = Class              #55            // java/lang/System
  #44 = NameAndType        #56:#57        // out:Ljava/io/PrintStream;
  #45 = Class              #50            // java/io/PrintStream
  #46 = NameAndType        #58:#59        // println:(Z)V
  #47 = Utf8               cn/itcast/jvm/t1/stringtable/Demo1_22
  #48 = Utf8               java/lang/Object
  #49 = Utf8               java/lang/String
  #50 = Utf8               java/io/PrintStream
  #51 = Utf8               append
  #52 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #53 = Utf8               toString
  #54 = Utf8               ()Ljava/lang/String;
  #55 = Utf8               java/lang/System
  #56 = Utf8               out
  #57 = Utf8               Ljava/io/PrintStream;
  #58 = Utf8               println
  #59 = Utf8               (Z)V
{
  public cn.itcast.jvm.t1.stringtable.Demo1_22();
    descriptor: ()V
    flags: 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 4: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcn/itcast/jvm/t1/stringtable/Demo1_22;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, 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: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
        36: aload_3
        37: aload         4
        39: if_acmpne     46
        42: iconst_1
        43: goto          47
        46: iconst_0
        47: invokevirtual #10                 // Method java/io/PrintStream.println:(Z)V
        50: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
        53: aload_3
        54: aload         5
        56: if_acmpne     63
        59: iconst_1
        60: goto          64
        63: iconst_0
        64: invokevirtual #10                 // Method java/io/PrintStream.println:(Z)V
        67: return
      LineNumberTable:
        line 11: 0
        line 12: 3
        line 13: 6
        line 14: 9
        line 15: 29
        line 16: 33
        line 17: 50
        line 18: 67
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      68     0  args   [Ljava/lang/String;
            3      65     1    s1   Ljava/lang/String;
            6      62     2    s2   Ljava/lang/String;
            9      59     3    s3   Ljava/lang/String;
           29      39     4    s4   Ljava/lang/String;
           33      35     5    s5   Ljava/lang/String;
      StackMapTable: number_of_entries = 4
        frame_type = 255 /* full_frame */
          offset_delta = 46
          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
          stack = [ class java/io/PrintStream ]
        frame_type = 255 /* full_frame */
          offset_delta = 0
          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
          stack = [ class java/io/PrintStream, int ]
        frame_type = 79 /* same_locals_1_stack_item */
          stack = [ class java/io/PrintStream ]
        frame_type = 255 /* full_frame */
          offset_delta = 0
          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
          stack = [ class java/io/PrintStream, int ]
}
SourceFile: "Demo1_22.java"

我们从中摘取最重要的一段：

 Code:
      stack=3, 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

上面每句的意思可参照<u>jvm指令手册</u>去进行阅读

     0: ldc           #2                  // String a
     ldc 把常量池中的项压入栈,即在Constant pool中找到#2,然后将其压入栈。也就是String a

astore_1 // astore_1 将引用类型或returnAddress类型值存入局部变量1
  也就是LocalVariableTable中的这一行
       3      65     1    s1   Ljava/lang/String;

其他类似，

在索引9的位置：

   9: new           #5                  // class java/lang/StringBuilder

这就是创建了一个StringBuilder对象，

 13: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V

"<init>":()V调用了无参构造方法

即

invokespecial 根据编译时类型来调用实例方法

   16: aload_1 // aload_1 从局部变量1中装载引用类型值,就是之前的String a

  17: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

调用了StringBuilder对象的append方法，

后面几句类似。

接着：

        24: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

调用了StringBuilder对象的toString方法。

综合下来：

这一句虚拟机底层就干了这个事，

String s4 = s1 + s2; // new StringBuilder().append("a").append("b").toString()

我们来看看StringBuilder对象的toString方法。

    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }

可以发现是创建了一个字符串对象：new String("ab")

而这一行：

 String s5 = "a" + "b";  // javac 在编译期间的优化，结果已经在编译期确定为ab

对应：

 29: ldc           #4                  // String ab
        31: astore        5

即直接拿到了#4，可在常量池中对应找下。也就是"ab"v这个字符串。

所以：

new StringBuilder().append("a").append("b").toString()  new String("ab")自然s3和s5都是从常量池中拿的"ab"
而s4是通过new StringBuilder().append("a").append("b").toString() // new String("ab")
    即在堆中的对象

可参照jvm指令手册进行理解，随手百度即可。

总结：

常量池中的字符串仅是符号，第一次用到时才变为对象
利用串池的机制，来避免重复创建字符串对象
字符串变量拼接的原理是 StringBuilder （1.8）
字符串常量拼接的原理是编译期优化

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