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jvm指令手册去进行阅读

     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

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