-extdirs[dirs] 覆蓋搜索安裝方式擴展的位置。擴展的缺省位置是 jrelibext。

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

示例：

下面也經典的StringBuilder代替String做字符串的例子。

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后綴。

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

}

1。contactWithStringNoLoopNoPara方法中，代碼里面是字符串拼接，貌似需要是用StringBuilder替換的好。其實在看了上面的反編譯結果后，已經自動組合成一個固定字符串了。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，因為使用到了變量，貌似需要是用StringBuilder替換的好。其實在看了上面的反編譯結果后，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，是循環拼接字符串，貌似需要是用StringBuilder替換的好。看了反編譯后，每個循環里面都各自生成了一個StringBuilder，并將StringBuilder.toString()防賦值給我們的Sring變量。而我們希望的是只生成一個StringBuilder對象。因此改為StringBuilder的好。循環的時候改為contactWithBufferLoop的方法最好。

4.contactWithBufferLoop方法中，是循環拼接字符串。也是我們預想的步驟在執行。