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Java - Disassembling and Decompiling
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  Viewing bytecode with javap

If you want to see the generated bytecode for a Java program, you can use the provided javap command to view it.

Assuming that we have the following Java source file:

package com.stackoverflow.documentation;

import org.springframework.stereotype.Service;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

@Service
public class HelloWorldService {

    public void sayHello() {
        System.out.println("Hello, World!");
    }

    private Object[] pvtMethod(List<String> strings) {
        return new Object[]{strings};
    }

    protected String tryCatchResources(String filename) throws IOException {
        try (InputStream inputStream = getClass().getResourceAsStream(filename)) {
            byte[] bytes = new byte[8192];
            int read = inputStream.read(bytes);
            return new String(bytes, 0, read);
        } catch (IOException | RuntimeException e) {
            e.printStackTrace();
            throw e;
        }
    }

    void stuff() {
        System.out.println("stuff");
    }
}

After compiling the source file, the most simple usage is:

cd <directory containing classes> (e.g. target/classes)
javap com/stackoverflow/documentation/SpringExample

Which produces the output

Compiled from "HelloWorldService.java"
public class com.stackoverflow.documentation.HelloWorldService {
  public com.stackoverflow.documentation.HelloWorldService();
  public void sayHello();
  protected java.lang.String tryCatchResources(java.lang.String) throws java.io.IOException;
  void stuff();
}

This lists all non-private methods in the class, but that is not particularly useful for most purposes. The following command is a lot more useful:

javap -p -c -s -constants -l -v com/stackoverflow/documentation/HelloWorldService

Which produces the output:

Classfile /Users/pivotal/IdeaProjects/stackoverflow-spring-docs/target/classes/com/stackoverflow/documentation/HelloWorldService.class
  Last modified Jul 22, 2016; size 2167 bytes
  MD5 checksum 6e33b5c292ead21701906353b7f06330
  Compiled from "HelloWorldService.java"
public class com.stackoverflow.documentation.HelloWorldService
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
    #1 = Methodref          #5.#60        // java/lang/Object."<init>":()V
    #2 = Fieldref           #61.#62       // java/lang/System.out:Ljava/io/PrintStream;
    #3 = String             #63           // Hello, World!
    #4 = Methodref          #64.#65       // java/io/PrintStream.println:(Ljava/lang/String;)V
    #5 = Class              #66           // java/lang/Object
    #6 = Methodref          #5.#67        // java/lang/Object.getClass:()Ljava/lang/Class;
    #7 = Methodref          #68.#69       // java/lang/Class.getResourceAsStream:(Ljava/lang/String;)Ljava/io/InputStream;
    #8 = Methodref          #70.#71       // java/io/InputStream.read:([B)I
    #9 = Class              #72           // java/lang/String
   #10 = Methodref          #9.#73        // java/lang/String."<init>":([BII)V
   #11 = Methodref          #70.#74       // java/io/InputStream.close:()V
   #12 = Class              #75           // java/lang/Throwable
   #13 = Methodref          #12.#76       // java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
   #14 = Class              #77           // java/io/IOException
   #15 = Class              #78           // java/lang/RuntimeException
   #16 = Methodref          #79.#80       // java/lang/Exception.printStackTrace:()V
   #17 = String             #55           // stuff
   #18 = Class              #81           // com/stackoverflow/documentation/HelloWorldService
   #19 = Utf8               <init>
   #20 = Utf8               ()V
   #21 = Utf8               Code
   #22 = Utf8               LineNumberTable
   #23 = Utf8               LocalVariableTable
   #24 = Utf8               this
   #25 = Utf8               Lcom/stackoverflow/documentation/HelloWorldService;
   #26 = Utf8               sayHello
   #27 = Utf8               pvtMethod
   #28 = Utf8               (Ljava/util/List;)[Ljava/lang/Object;
   #29 = Utf8               strings
   #30 = Utf8               Ljava/util/List;
   #31 = Utf8               LocalVariableTypeTable
   #32 = Utf8               Ljava/util/List<Ljava/lang/String;>;
   #33 = Utf8               Signature
   #34 = Utf8               (Ljava/util/List<Ljava/lang/String;>;)[Ljava/lang/Object;
   #35 = Utf8               tryCatchResources
   #36 = Utf8               (Ljava/lang/String;)Ljava/lang/String;
   #37 = Utf8               bytes
   #38 = Utf8               [B
   #39 = Utf8               read
   #40 = Utf8               I
   #41 = Utf8               inputStream
   #42 = Utf8               Ljava/io/InputStream;
   #43 = Utf8               e
   #44 = Utf8               Ljava/lang/Exception;
   #45 = Utf8               filename
   #46 = Utf8               Ljava/lang/String;
   #47 = Utf8               StackMapTable
   #48 = Class              #81           // com/stackoverflow/documentation/HelloWorldService
   #49 = Class              #72           // java/lang/String
   #50 = Class              #82           // java/io/InputStream
   #51 = Class              #75           // java/lang/Throwable
   #52 = Class              #38           // "[B"
   #53 = Class              #83           // java/lang/Exception
   #54 = Utf8               Exceptions
   #55 = Utf8               stuff
   #56 = Utf8               SourceFile
   #57 = Utf8               HelloWorldService.java
   #58 = Utf8               RuntimeVisibleAnnotations
   #59 = Utf8               Lorg/springframework/stereotype/Service;
   #60 = NameAndType        #19:#20       // "<init>":()V
   #61 = Class              #84           // java/lang/System
   #62 = NameAndType        #85:#86       // out:Ljava/io/PrintStream;
   #63 = Utf8               Hello, World!
   #64 = Class              #87           // java/io/PrintStream
   #65 = NameAndType        #88:#89       // println:(Ljava/lang/String;)V
   #66 = Utf8               java/lang/Object
   #67 = NameAndType        #90:#91       // getClass:()Ljava/lang/Class;
   #68 = Class              #92           // java/lang/Class
   #69 = NameAndType        #93:#94       // getResourceAsStream:(Ljava/lang/String;)Ljava/io/InputStream;
   #70 = Class              #82           // java/io/InputStream
   #71 = NameAndType        #39:#95       // read:([B)I
   #72 = Utf8               java/lang/String
   #73 = NameAndType        #19:#96       // "<init>":([BII)V
   #74 = NameAndType        #97:#20       // close:()V
   #75 = Utf8               java/lang/Throwable
   #76 = NameAndType        #98:#99       // addSuppressed:(Ljava/lang/Throwable;)V
   #77 = Utf8               java/io/IOException
   #78 = Utf8               java/lang/RuntimeException
   #79 = Class              #83           // java/lang/Exception
   #80 = NameAndType        #100:#20      // printStackTrace:()V
   #81 = Utf8               com/stackoverflow/documentation/HelloWorldService
   #82 = Utf8               java/io/InputStream
   #83 = Utf8               java/lang/Exception
   #84 = Utf8               java/lang/System
   #85 = Utf8               out
   #86 = Utf8               Ljava/io/PrintStream;
   #87 = Utf8               java/io/PrintStream
   #88 = Utf8               println
   #89 = Utf8               (Ljava/lang/String;)V
   #90 = Utf8               getClass
   #91 = Utf8               ()Ljava/lang/Class;
   #92 = Utf8               java/lang/Class
   #93 = Utf8               getResourceAsStream
   #94 = Utf8               (Ljava/lang/String;)Ljava/io/InputStream;
   #95 = Utf8               ([B)I
   #96 = Utf8               ([BII)V
   #97 = Utf8               close
   #98 = Utf8               addSuppressed
   #99 = Utf8               (Ljava/lang/Throwable;)V
  #100 = Utf8               printStackTrace
{
  public com.stackoverflow.documentation.HelloWorldService();
    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 10: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/stackoverflow/documentation/HelloWorldService;

  public void sayHello();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Hello, World!
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 13: 0
        line 14: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  this   Lcom/stackoverflow/documentation/HelloWorldService;

  private java.lang.Object[] pvtMethod(java.util.List<java.lang.String>);
    descriptor: (Ljava/util/List;)[Ljava/lang/Object;
    flags: ACC_PRIVATE
    Code:
      stack=4, locals=2, args_size=2
         0: iconst_1
         1: anewarray     #5                  // class java/lang/Object
         4: dup
         5: iconst_0
         6: aload_1
         7: aastore
         8: areturn
      LineNumberTable:
        line 17: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  this   Lcom/stackoverflow/documentation/HelloWorldService;
            0       9     1 strings   Ljava/util/List;
      LocalVariableTypeTable:
        Start  Length  Slot  Name   Signature
            0       9     1 strings   Ljava/util/List<Ljava/lang/String;>;
    Signature: #34                          // (Ljava/util/List<Ljava/lang/String;>;)[Ljava/lang/Object;

  protected java.lang.String tryCatchResources(java.lang.String) throws java.io.IOException;
    descriptor: (Ljava/lang/String;)Ljava/lang/String;
    flags: ACC_PROTECTED
    Code:
      stack=5, locals=10, args_size=2
         0: aload_0
         1: invokevirtual #6                  // Method java/lang/Object.getClass:()Ljava/lang/Class;
         4: aload_1
         5: invokevirtual #7                  // Method java/lang/Class.getResourceAsStream:(Ljava/lang/String;)Ljava/io/InputStream;
         8: astore_2
         9: aconst_null
        10: astore_3
        11: sipush        8192
        14: newarray       byte
        16: astore        4
        18: aload_2
        19: aload         4
        21: invokevirtual #8                  // Method java/io/InputStream.read:([B)I
        24: istore        5
        26: new           #9                  // class java/lang/String
        29: dup
        30: aload         4
        32: iconst_0
        33: iload         5
        35: invokespecial #10                 // Method java/lang/String."<init>":([BII)V
        38: astore        6
        40: aload_2
        41: ifnull        70
        44: aload_3
        45: ifnull        66
        48: aload_2
        49: invokevirtual #11                 // Method java/io/InputStream.close:()V
        52: goto          70
        55: astore        7
        57: aload_3
        58: aload         7
        60: invokevirtual #13                 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
        63: goto          70
        66: aload_2
        67: invokevirtual #11                 // Method java/io/InputStream.close:()V
        70: aload         6
        72: areturn
        73: astore        4
        75: aload         4
        77: astore_3
        78: aload         4
        80: athrow
        81: astore        8
        83: aload_2
        84: ifnull        113
        87: aload_3
        88: ifnull        109
        91: aload_2
        92: invokevirtual #11                 // Method java/io/InputStream.close:()V
        95: goto          113
        98: astore        9
       100: aload_3
       101: aload         9
       103: invokevirtual #13                 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
       106: goto          113
       109: aload_2
       110: invokevirtual #11                 // Method java/io/InputStream.close:()V
       113: aload         8
       115: athrow
       116: astore_2
       117: aload_2
       118: invokevirtual #16                 // Method java/lang/Exception.printStackTrace:()V
       121: aload_2
       122: athrow
      Exception table:
         from    to  target type
            48    52    55   Class java/lang/Throwable
            11    40    73   Class java/lang/Throwable
            11    40    81   any
            91    95    98   Class java/lang/Throwable
            73    83    81   any
             0    70   116   Class java/io/IOException
             0    70   116   Class java/lang/RuntimeException
            73   116   116   Class java/io/IOException
            73   116   116   Class java/lang/RuntimeException
      LineNumberTable:
        line 21: 0
        line 22: 11
        line 23: 18
        line 24: 26
        line 25: 40
        line 21: 73
        line 25: 81
        line 26: 117
        line 27: 121



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