java命令之javap

javap可以查看java编译器为我们生成的字节码，其使用方法可查阅帮助文档

>javap --help

用法: javap <options> <classes>
其中, 可能的选项包括:
-help --help -? 输出此用法消息
-version 版本信息
-v -verbose 输出附加信息
-l 输出行号和本地变量表
-public 仅显示公共类和成员
-protected 显示受保护的/公共类和成员
-package 显示程序包/受保护的/公共类和成员 (默认)
-p -private 显示所有类和成员
-c 对代码进行反汇编
-s 输出内部类型签名
-sysinfo 显示正在处理的类的系统信息 (路径, 大小, 日期, MD5 散列)
-constants 显示最终常量
-classpath <path> 指定查找用户类文件的位置
-cp <path> 指定查找用户类文件的位置
-bootclasspath <path> 覆盖引导类文件的位置

javap实战
源码如下：

public class test1 {
    static String s = "A";

    public static void main(String[] args) {
        System.out.println(test());
        System.out.println(s);
    }

    private static String test() {
        try {
            System.out.println("A");
            return s = "A";
        } finally {
            System.out.println("B");
            s = "B";
        }
    }
}

执行javap命令javap -c test1

javap -c test1
Compiled from "test1.java"
public class test1 {
  static java.lang.String s;

  public test1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: invokestatic  #3                  // Method test:()Ljava/lang/String;
       6: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       9: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      12: getstatic     #5                  // Field s:Ljava/lang/String;
      15: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      18: return
  static {};
    Code:
       0: ldc           #6                  // String A
       2: putstatic     #5                  // Field s:Ljava/lang/String;
       5: return
}

执行javap -c -v test1查看更多信息

javap -c -v test1
Classfile /F:/sas/Test/target/classes/test1.class
  Last modified 2017-12-9; size 836 bytes
  MD5 checksum 8c21be20560aefeec48520f923d513d2
  Compiled from "test1.java"
public class test1
  minor version: 0
  major version: 50
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #9.#30         // java/lang/Object."<init>":()V
   #2 = Fieldref           #31.#32        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Methodref          #8.#33         // test1.test:()Ljava/lang/String;
   #4 = Methodref          #34.#35        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Fieldref           #8.#36         // test1.s:Ljava/lang/String;
   #6 = String             #37            // A
   #7 = String             #38            // B
   #8 = Class              #39            // test1
   #9 = Class              #40            // java/lang/Object
  #10 = Utf8               s
  #11 = Utf8               Ljava/lang/String;
  #12 = Utf8               <init>
  #13 = Utf8               ()V
  #14 = Utf8               Code
  #15 = Utf8               LineNumberTable
  #16 = Utf8               LocalVariableTable
  #17 = Utf8               this
  #18 = Utf8               Ltest1;
  #19 = Utf8               main
  #20 = Utf8               ([Ljava/lang/String;)V
  #21 = Utf8               args
  #22 = Utf8               [Ljava/lang/String;
  #23 = Utf8               test
  #24 = Utf8               ()Ljava/lang/String;
  #25 = Utf8               StackMapTable
  #26 = Class              #41            // java/lang/Throwable
  #27 = Utf8               <clinit>
  #28 = Utf8               SourceFile
  #29 = Utf8               test1.java
  #30 = NameAndType        #12:#13        // "<init>":()V
  #31 = Class              #42            // java/lang/System
  #32 = NameAndType        #43:#44        // out:Ljava/io/PrintStream;
  #33 = NameAndType        #23:#24        // test:()Ljava/lang/String;
  #34 = Class              #45            // java/io/PrintStream
  #35 = NameAndType        #46:#47        // println:(Ljava/lang/String;)V
  #36 = NameAndType        #10:#11        // s:Ljava/lang/String;
  #37 = Utf8               A
  #38 = Utf8               B
  #39 = Utf8               test1
  #40 = Utf8               java/lang/Object
  #41 = Utf8               java/lang/Throwable
  #42 = Utf8               java/lang/System
  #43 = Utf8               out
  #44 = Utf8               Ljava/io/PrintStream;
  #45 = Utf8               java/io/PrintStream
  #46 = Utf8               println
  #47 = Utf8               (Ljava/lang/String;)V
{
  static java.lang.String s;
    descriptor: Ljava/lang/String;
    flags: ACC_STATIC

  public test1();
    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 2: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ltest1;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: invokestatic  #3                  // Method test:()Ljava/lang/String;
         6: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         9: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        12: getstatic     #5                  // Field s:Ljava/lang/String;
        15: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        18: return
      LineNumberTable:
        line 6: 0
        line 7: 9
        line 8: 18
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      19     0  args   [Ljava/lang/String;

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #6                  // String A
         2: putstatic     #5                  // Field s:Ljava/lang/String;
         5: return
      LineNumberTable:
        line 3: 0
}
SourceFile: "test1.java"

其中栈和局部变量操作参见 Java虚拟机--字节码指令集

查阅资料
Java中return与finally的执行顺序分析（根据字节码分析）
javap 学习日记
Java finally语句到底是在return之前还是之后执行？
finally执行顺序面试题