javap的基本用法

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參考:http://www.cnblogs.com/beautiful-code/p/6424977.html

 

javap是JDK內建的反組譯碼器，可以查看java編譯器為我們產生的位元組碼。通過它，我們可以對照原始碼和位元組碼，從而瞭解很多編譯器內部的工作。
文法：

　

 

 

英文說明：

C:\>javap -help
Usage: javap <options> <classes>...

where options include:
   -c                        Disassemble the code
   -classpath <pathlist>     Specify where to find user class files
   -extdirs <dirs>           Override location of installed extensions
   -help                     Print this usage message
   -J<flag>                  Pass <flag> directly to the runtime system
   -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)
   -private                  Show all classes and members
   -s                        Print internal type signatures
   -bootclasspath <pathlist> Override location of class files loaded
                             by the bootstrap class loader
   -verbose                  Print stack size, number of locals and args for methods
                             If verifying, print reasons for failure

 

 

樣本：

 

下面也經典的StringBuilder代替String做字串的例子。

public class JAVAPTest {      public static void main(String[] args) {        }        public static String contactWithStringNoLoopNoPara() {          String s = "This is " + " my " + "first JAVAP test code.";          return s;      }            public static String contactWithStringNoLoop(int count) {          String s = "This is " + " my " + count + "th JAVAP test code.";          return s;      }            public static String contactWithStringLoop(int count) {          String s = "";          for (int i = 0; i < count; i++) {              s += i;          }          return s;      }        public static String contactWithStringBufferLoop(int count) {          StringBuffer sb = new StringBuffer();          for (int i = 0; i < count; i++) {              sb.append(i);          }          return sb.toString();      }  } 

 

 

先編譯：javac JAVAPTest.java

執行反編譯：javap -c JAVAPTest         //注意這個地方不需要class尾碼。

 

結果如下：

Compiled from "JAVAPTest.java"  public class JAVAPTest extends java.lang.Object{  public JAVAPTest();    Code:     0:   aload_0     1:   invokespecial   #1; //Method java/lang/Object."<init>":()V     4:   return    public static void main(java.lang.String[]);    Code:     0:   return    public static java.lang.String contactWithStringNoLoopNoPara();    Code:     0:   ldc     #2; //String This is  my first JAVAP test code.     2:   astore_0     3:   aload_0     4:   areturn    public static java.lang.String contactWithStringNoLoop(int);    Code:     0:   new     #3; //class java/lang/StringBuilder     3:   dup     4:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V     7:   ldc     #5; //String This is  my     9:   invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;     12:  iload_0     13:  invokevirtual   #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;     16:  ldc     #8; //String th JAVAP test code.     18:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;     21:  invokevirtual   #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;     24:  astore_1     25:  aload_1     26:  areturn    public static java.lang.String contactWithStringLoop(int);    Code:     0:   ldc     #10; //String     2:   astore_1     3:   iconst_0     4:   istore_2     5:   iload_2     6:   iload_0     7:   if_icmpge       35     10:  new     #3; //class java/lang/StringBuilder     13:  dup     14:  invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V     17:  aload_1     18:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;     21:  iload_2     22:  invokevirtual   #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;     25:  invokevirtual   #9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;     28:  astore_1     29:  iinc    2, 1     32:  goto    5     35:  aload_1     36:  areturn    public static java.lang.String contactWithStringBufferLoop(int);    Code:     0:   new     #11; //class java/lang/StringBuffer     3:   dup     4:   invokespecial   #12; //Method java/lang/StringBuffer."<init>":()V     7:   astore_1     8:   iconst_0     9:   istore_2     10:  iload_2     11:  iload_0     12:  if_icmpge       27     15:  aload_1     16:  iload_2     17:  invokevirtual   #13; //Method java/lang/StringBuffer.append:(I)Ljava/lang/StringBuffer;     20:  pop     21:  iinc    2, 1     24:  goto    10     27:  aload_1     28:  invokevirtual   #14; //Method java/lang/StringBuffer.toString:()Ljava/lang/String;     31:  areturn    }

有這個結果我們可以知道。

1。contactWithStringNoLoopNoPara方法中，代碼裡面是字串拼接，貌似需要是用StringBuilder替換的好。其實在看了上面的反編譯結果後，已經自動組合成一個固定字串了。因此完全沒有必要使用StringBuilder。

0:   ldc     #2; //String This is  my first JAVAP test code.

 

2。contactWithStringNoLoop方法中，因為使用到了變數，貌似需要是用StringBuilder替換的好。其實在看了上面的反編譯結果後，已經自動使用了StringBuilder。所以代碼也沒有必要使用StringBuilder。

3. contactWithStringLoop方法中，是迴圈拼接字串，貌似需要是用StringBuilder替換的好。看了反編譯後，每個迴圈裡面都各自產生了一個StringBuilder，並將StringBuilder.toString()防賦值給我們的Sring變數。而我們希望的是只產生一個StringBuilder對象。因此改為StringBuilder的好。迴圈的時候改為contactWithBufferLoop的方法最好。

4.contactWithBufferLoop方法中，是迴圈拼接字串。也是我們預想的步驟在執行。

 

javap的基本用法