# Disassembling and Decompiling
# 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
LocalVariableTable:
Start Length Slot Name Signature
18 55 4 bytes [B
26 47 5 read I
9 107 2 inputStream Ljava/io/InputStream;
117 6 2 e Ljava/lang/Exception;
0 123 0 this Lcom/stackoverflow/documentation/HelloWorldService;
0 123 1 filename Ljava/lang/String;
StackMapTable: number_of_entries = 9
frame_type = 255 /* full_frame */
offset_delta = 55
locals = [ class com/stackoverflow/documentation/HelloWorldService, class java/lang/String, class java/io/InputStream, class java/lang/Throwable, class "[B", int, class java/lang/String ]
stack = [ class java/lang/Throwable ]
frame_type = 10 /* same */
frame_type = 3 /* same */
frame_type = 255 /* full_frame */
offset_delta = 2
locals = [ class com/stackoverflow/documentation/HelloWorldService, class java/lang/String, class java/io/InputStream, class java/lang/Throwable ]
stack = [ class java/lang/Throwable ]
frame_type = 71 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
frame_type = 255 /* full_frame */
offset_delta = 16
locals = [ class com/stackoverflow/documentation/HelloWorldService, class java/lang/String, class java/io/InputStream, class java/lang/Throwable, top, top, top, top, class java/lang/Throwable ]
stack = [ class java/lang/Throwable ]
frame_type = 10 /* same */
frame_type = 3 /* same */
frame_type = 255 /* full_frame */
offset_delta = 2
locals = [ class com/stackoverflow/documentation/HelloWorldService, class java/lang/String ]
stack = [ class java/lang/Exception ]
Exceptions:
throws java.io.IOException
void stuff();
descriptor: ()V
flags:
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #17 // String stuff
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 32: 0
line 33: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/stackoverflow/documentation/HelloWorldService;
}
SourceFile: "HelloWorldService.java"
RuntimeVisibleAnnotations:
0: #59()
# Syntax
- javap [options]
# Parameters
| Name | Description |
|---|---|
<classes> | List of classes to disassemble. Can be in either package1.package2.Classname format, or package1/package2/Classname format. Do not include the .class extension. |
-help, --help, -? | Print this usage message |
-version | Version information |
-v, -verbose | Print additional information |
-l | Print line number and local variable tables |
-public | Show only public classes and members |
-protected | Show protected/public classes and members |
-package | Show package/protected/public classes and members (default) |
-p, -private | Show all classes and members |
-c | Disassemble the code |
-s | Print internal type signatures |
-sysinfo | Show system info (path, size, date, MD5 hash) of class being processed |
-constants | Show final constants |
-classpath <path> | Specify where to find user class files |
-cp <path> | Specify where to find user class files |
-bootclasspath <path> | Override location of bootstrap class files |