Many times, we need to understand what a compiler is doing under the hood. How the java statements we are writing, will be reordered and executed. Also, we need to see the byte code for learning purposes also, I do it seldom. In this tutorial, I am giving an example of how to generate the byte code for a class file in java.
To demonstrate the example, I am using the java file created for my other tutorial related to automatic resource management in java 7.
1. Compile Java File using the command javac
This is optional because you might have the .class file already.
prompt > javac C://temp/java/test/ResourceManagementInJava7.javaThis will generate the .class file ResourceManagementInJava7.class.
2. Execute javap Command and Redirect Output to .bc File
C:>javap -c C://temp/java/test/ResourceManagementInJava7.class > C://temp/java/test/bytecode.bc
Let’s look at the command run on the prompt.
A file bytecode.bc file will be generated at a given location. It will be something like this:
public class com.howtodoinjava.java7.tryCatch.ResourceManagementInJava7 {
public com.howtodoinjava.java7.tryCatch.ResourceManagementInJava7();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: new #2 // class java/io/BufferedReader
3: dup
4: new #3 // class java/io/FileReader
7: dup
8: ldc #4 // String C:/temp/test1.txt
10: invokespecial #5 // Method java/io/FileReader."<init>":(Ljava/lang/String;)V
13: invokespecial #6 // Method java/io/BufferedReader."<init>":(Ljava/io/Reader;)V
16: astore_1
17: aconst_null
18: astore_2
19: new #2 // class java/io/BufferedReader
22: dup
23: new #3 // class java/io/FileReader
26: dup
27: ldc #7 // String C:/temp/test2.txt
29: invokespecial #5 // Method java/io/FileReader."<init>":(Ljava/lang/String;)V
32: invokespecial #6 // Method java/io/BufferedReader."<init>":(Ljava/io/Reader;)V
35: astore_3
36: aconst_null
37: astore 4
39: new #2 // class java/io/BufferedReader
42: dup
43: new #3 // class java/io/FileReader
46: dup
47: ldc #8 // String C:/temp/test3.txt
49: invokespecial #5 // Method java/io/FileReader."<init>":(Ljava/lang/String;)V
52: invokespecial #6 // Method java/io/BufferedReader."<init>":(Ljava/io/Reader;)V
55: astore 5
57: aconst_null
58: astore 6
60: aload 5
62: ifnull 138
65: aload 6
67: ifnull 90
70: aload 5
72: invokevirtual #9 // Method java/io/BufferedReader.close:()V
75: goto 138
78: astore 7
80: aload 6
82: aload 7
84: invokevirtual #11 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
87: goto 138
90: aload 5
92: invokevirtual #9 // Method java/io/BufferedReader.close:()V
95: goto 138
98: astore 8
100: aload 5
102: ifnull 135
105: aload 6
107: ifnull 130
110: aload 5
112: invokevirtual #9 // Method java/io/BufferedReader.close:()V
115: goto 135
118: astore 9
120: aload 6
122: aload 9
124: invokevirtual #11 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
127: goto 135
130: aload 5
132: invokevirtual #9 // Method java/io/BufferedReader.close:()V
135: aload 8
137: athrow
138: aload_3
139: ifnull 219
142: aload 4
144: ifnull 166
147: aload_3
148: invokevirtual #9 // Method java/io/BufferedReader.close:()V
151: goto 219
154: astore 5
156: aload 4
158: aload 5
160: invokevirtual #11 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
163: goto 219
166: aload_3
167: invokevirtual #9 // Method java/io/BufferedReader.close:()V
170: goto 219
173: astore 5
175: aload 5
177: astore 4
179: aload 5
181: athrow
182: astore 10
184: aload_3
185: ifnull 216
188: aload 4
190: ifnull 212
193: aload_3
194: invokevirtual #9 // Method java/io/BufferedReader.close:()V
197: goto 216
200: astore 11
202: aload 4
204: aload 11
206: invokevirtual #11 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
209: goto 216
212: aload_3
213: invokevirtual #9 // Method java/io/BufferedReader.close:()V
216: aload 10
218: athrow
219: aload_1
220: ifnull 290
223: aload_2
224: ifnull 243
227: aload_1
228: invokevirtual #9 // Method java/io/BufferedReader.close:()V
231: goto 290
234: astore_3
235: aload_2
236: aload_3
237: invokevirtual #11 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
240: goto 290
243: aload_1
244: invokevirtual #9 // Method java/io/BufferedReader.close:()V
247: goto 290
250: astore_3
251: aload_3
252: astore_2
253: aload_3
254: athrow
255: astore 12
257: aload_1
258: ifnull 287
261: aload_2
262: ifnull 283
265: aload_1
266: invokevirtual #9 // Method java/io/BufferedReader.close:()V
269: goto 287
272: astore 13
274: aload_2
275: aload 13
277: invokevirtual #11 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
280: goto 287
283: aload_1
284: invokevirtual #9 // Method java/io/BufferedReader.close:()V
287: aload 12
289: athrow
290: goto 298
293: astore_1
294: aload_1
295: invokevirtual #13 // Method java/io/IOException.printStackTrace:()V
298: return
Exception table:
from to target type
70 75 78 Class java/lang/Throwable
110 115 118 Class java/lang/Throwable
98 100 98 any
147 151 154 Class java/lang/Throwable
39 138 173 Class java/lang/Throwable
39 138 182 any
193 197 200 Class java/lang/Throwable
173 184 182 any
227 231 234 Class java/lang/Throwable
19 219 250 Class java/lang/Throwable
19 219 255 any
265 269 272 Class java/lang/Throwable
250 257 255 any
0 290 293 Class java/io/IOException
}Happy Learning !!
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