查看Java字节码内容的几种方式

1、javap

通过javac和javap查看：先通过javac将.java文件编译成.class字节码文件，然后通过javap -verbose分析字节码。

(base) tinytongtongdeMacBook-Pro% javac JVMRunTimeStorageTest.java
(base) tinytongtongdeMacBook-Pro% javap -verbose JVMRunTimeStorageTest

这样你就能看到你的字节码信息了。

示例如下：

Classfile /Users/wangjianzhou/Documents/workspace/java/JavaStudy/src/com/tinytongtong/javastudy/jvm/JVMRunTimeStorageTest.class
  Last modified Mar 16, 2022; size 657 bytes
  MD5 checksum a13778df0b853b554f2a5da237c53672
  Compiled from "JVMRunTimeStorageTest.java"
public class com.tinytongtong.javastudy.jvm.JVMRunTimeStorageTest
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#21         // java/lang/Object."<init>":()V
   #2 = Double             2.0d
   #4 = Class              #22            // com/tinytongtong/javastudy/jvm/JVMRunTimeStorageTest
   #5 = Class              #23            // java/lang/Object
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               main
  #11 = Utf8               ([Ljava/lang/String;)V
  #12 = Utf8               add
  #13 = Utf8               (I)I
  #14 = Utf8               add1
  #15 = Utf8               add2
  #16 = Utf8               ()I
  #17 = Utf8               foreach
  #18 = Utf8               StackMapTable
  #19 = Utf8               SourceFile
  #20 = Utf8               JVMRunTimeStorageTest.java
  #21 = NameAndType        #6:#7          // "<init>":()V
  #22 = Utf8               com/tinytongtong/javastudy/jvm/JVMRunTimeStorageTest
  #23 = Utf8               java/lang/Object
{
  public com.tinytongtong.javastudy.jvm.JVMRunTimeStorageTest();
    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 8: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 12: 0

  public static int add(int);
    descriptor: (I)I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1
         1: istore_1
         2: iconst_2
         3: istore_2
         4: iload_1
         5: iload_2
         6: iadd
         7: iload_0
         8: iadd
         9: ireturn
      LineNumberTable:
        line 28: 0
        line 29: 2
        line 30: 4

  public static int add1(int);
    descriptor: (I)I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=5, args_size=1
         0: lconst_1
         1: lstore_1
         2: ldc2_w        #2                  // double 2.0d
         5: dstore_3
         6: lload_1
         7: l2d
         8: dload_3
         9: dadd
        10: iload_0
        11: i2d
        12: dadd
        13: d2i
        14: ireturn
      LineNumberTable:
        line 42: 0
        line 43: 2
        line 44: 6

  public static int add2();
    descriptor: ()I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=0
         0: iconst_1
         1: istore_0
         2: iconst_2
         3: istore_1
         4: iload_0
         5: iload_1
         6: iadd
         7: istore_2
         8: iload_2
         9: bipush        10
        11: iadd
        12: ireturn
      LineNumberTable:
        line 55: 0
        line 56: 2
        line 57: 4
        line 58: 8

  public static int foreach(int);
    descriptor: (I)I
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_0
         1: istore_1
         2: iconst_0
         3: istore_2
         4: iload_2
         5: iload_0
         6: if_icmpge     19
         9: iload_1
        10: iload_2
        11: iadd
        12: istore_1
        13: iinc          2, 1
        16: goto          4
        19: iload_1
        20: ireturn
      LineNumberTable:
        line 70: 0
        line 71: 2
        line 72: 9
        line 71: 13
        line 74: 19
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 4
          locals = [ int, int ]
        frame_type = 250 /* chop */
          offset_delta = 14
}
SourceFile: "JVMRunTimeStorageTest.java"

demo-JVMRunTimeStorageTest

2、ASM Bytecode Outline 插件

给IDE安装ASM Bytecode Outline插件。

安装好之后，重启IDE。

接着选择对应的java文件，右键 --> Show Bytecode outline，可以看到ASM的输出，如下图所示

3、jclasslib Bytecode Viewer

jclasslib Bytecode Viewer

我们可以通过这个插件，以可视化的方式查看字节码中的信息，比如方法Code表中的局部变量表信息。如下图所示：

打开对应的java文件，View --> Show Bytecode With Jclasslib

我们可以看到局部变量表的长度。