怎么使用IDEA查看java文件编译后的字节码内容

首先编写一个java类 StringDemo1.java

public class StringDemo1 {
   public static void main(String[] args) {
       String str1 = "aaa" + "bbb";
       System.out.println(str1);
       String str2 = "ccc";
       str2 += "ddd";
       System.out.println(str2);
   }
}

经过编译后，生成 StringDemo1.class文件

使用IDEA查看编译之后的文件内容：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package jdk.java.lang.string;
public class StringDemo1 {
   public StringDemo1() {
   }
   public static void main(String[] args) {
       String str1 = "aaabbb";
       System.out.println(str1);
       String str2 = "ccc";
       str2 = str2 + "ddd";
       System.out.println(str2);
   }
}

可以看出内容已经是经过IDEA反编译之后的了，并不能看到字节码内容。

使用jclasslib反编译工具

在IDEA中搜索插件jclasslib bytecode viewer，然后进行安装，重启IDEA。

再次打开StringDemo1.java文件，在IDEA菜单栏上打开View -> Show Bytecode，即可以看到字节码文件内容:

// class version 52.0 (52)
// access flags 0x21
public class jdk/java/lang/string/StringDemo1 {
 // compiled from: StringDemo1.java
 // access flags 0x1
 public <init>()V
  L0
   LINENUMBER 8 L0
   ALOAD 0
   INVOKESPECIAL java/lang/Object.<init> ()V
   RETURN
  L1
   LOCALVARIABLE this Ljdk/java/lang/string/StringDemo1; L0 L1 0
   MAXSTACK = 1
   MAXLOCALS = 1
 // access flags 0x9
 public static main([Ljava/lang/String;)V
  L0
   LINENUMBER 11 L0
   LDC "aaabbb"
   ASTORE 1
  L1
   LINENUMBER 12 L1
   GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
   ALOAD 1
   INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  L2
   LINENUMBER 15 L2
   LDC "ccc"
   ASTORE 2
  L3
   LINENUMBER 16 L3
   NEW java/lang/StringBuilder
   DUP
   INVOKESPECIAL java/lang/StringBuilder.<init> ()V
   ALOAD 2
   INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
   LDC "ddd"
   INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
   INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
   ASTORE 2
  L4
   LINENUMBER 17 L4
   GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
   ALOAD 2
   INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  L5
   LINENUMBER 18 L5
   RETURN
  L6
   LOCALVARIABLE args [Ljava/lang/String; L0 L6 0
   LOCALVARIABLE str1 Ljava/lang/String; L1 L6 1
   LOCALVARIABLE str2 Ljava/lang/String; L3 L6 2
   MAXSTACK = 2
   MAXLOCALS = 3
}

JDK版本映射

JDK版本	class版本
J2SE 8	52
J2SE 7	51
J2SE 6.0	50
J2SE 5.0	49
JDK 1.4	48
JDK 1.3	47
JDK 1.2	46
JDK 1.1	45

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