无限递归的构造器和javap使用指南

无限递归的构造器和javap使用指南

public class ConstructorRecursion {

ConstructorRecursion rc;

{

rc = newConstructorRecursion();

}

public ConstructorRecursion(){

System.out.println("noparameter constructor");

}

public static void main(String[] args){

ConstructorRecursion rc = new ConstructorRecursion();

}

}

Exceptionin thread "main" java.lang.*Error

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

at ConstructorRecursion.<init>(ConstructorRecursion.java:5)

表面上看，上面程序没有任何问题，ConstructorRecursion类的构造器中没有任何代码，只是单纯一句输出。但不要忘记了，不管是定义实例变量时指定的初始值，还是在非静态初始化块中执行的初始化操作，最终都将提取到构造器中执行。因为上面代码递归调用了ConstructorRecursion类的狗仔去哦，所以抛出了*Error错误。

这个程序给出的教训是：无论如何不要导致构造器产生递归调用。也就是说，应该：

尽量不要在定义实例变量时指定实例变量的值为当前类的实例；

尽量不要初始化块中创建当前类的实例；

尽量不要在构造器内调用本构造器创建Java对象；

C:\Documents andSettings\mz50947\workspace\TestProject\src>javap ConstructorRe

ursion

Compiled from"ConstructorRecursion.java"

public classConstructorRecursion extends java.lang.Object{

ConstructorRecursion rc;

public ConstructorRecursion();

public static voidmain(java.lang.String[]);

}

使用JAVAP可以查看java编译器生成的字节码，通过比较字节码和源代码，可以发现很多的问题。

C:\Documents andSettings\mz50947\workspace\TestProject\src>javac -g Constructor

Recursion.java

08/08/2013  02:50 PM               645 ConstructorRecursion.class

08/08/2013  02:34 PM               304 ConstructorRecursion.java

不带参数的javap将打印类的public信息，包括成员和方法。

C:\Documents andSettings\mz50947\workspace\TestProject\src>javap -c Constructor

Recursion

Compiled from"ConstructorRecursion.java"

public classConstructorRecursion extends java.lang.Object{

ConstructorRecursionrc;

publicConstructorRecursion();

Code:

0:  aload_0

1:  invokespecial   #1; //Methodjava/lang/Object."<init>":()V

4:  aload_0

5:  new     #2; //classConstructorRecursion

8:  dup

9:  invokespecial   #3; //Method"<init>":()V

12: putfield        #4; //Fieldrc:LConstructorRecursion;

15: getstatic       #5; //Fieldjava/lang/System.out:Ljava/io/PrintStream;

18: ldc     #6; //String no parameterconstructor

20: invokevirtual   #7; //Methodjava/io/PrintStream.println:(Ljava/lang/Str

ing;)V

23: return

public static voidmain(java.lang.String[]);

Code:

0:  new     #2; //classConstructorRecursion

3:  dup

4:   invokespecial  #3; //Method "<init>":()V

7:  astore_1

8:  return

}

C:\Documents andSettings\mz50947\workspace\TestProject\src>javap -l Constructor

Recursion

Compiled from"ConstructorRecursion.java"

public classConstructorRecursion extends java.lang.Object{

ConstructorRecursionrc;

publicConstructorRecursion();

LineNumberTable:

line 7: 0

line 5: 4

line 8: 15

line 9: 23

LocalVariableTable:

Start Length  Slot  Name  Signature

0     24      0    this      LConstructorRecursion;

public static voidmain(java.lang.String[]);

LineNumberTable:

line 11: 0

line 12: 8

LocalVariableTable:

Start Length  Slot  Name  Signature

0     9      0    args      [Ljava/lang/String;

8     1      1    rc      LConstructorRecursion;

}

C:\Documents andSettings\mz50947\workspace\TestProject\src>javap -c Constructor

Recursion

Compiled from"ConstructorRecursion.java"

public classConstructorRecursion extends java.lang.Object{

ConstructorRecursionrc;

public ConstructorRecursion();

Code:

0:  aload_0

1:  invokespecial   #1; //Methodjava/lang/Object."<init>":()V

4:  aload_0

5:  new     #2; //classConstructorRecursion

8:  dup

9:  invokespecial   #3; //Method"<init>":()V

12: putfield        #4; //Fieldrc:LConstructorRecursion;

15: getstatic       #5; //Fieldjava/lang/System.out:Ljava/io/PrintStream;

18: ldc     #6; //String no parameterconstructor

20: invokevirtual   #7; //Methodjava/io/PrintStream.println:(Ljava/lang/Str

ing;)V

23: return

public static voidmain(java.lang.String[]);

Code:

0:  new     #2; //classConstructorRecursion

3:  dup

4:  invokespecial   #3; //Method"<init>":()V

7:  astore_1

8:  return

}

The javap command iscalled the Java “disassembler”because it takes apart class files andtells you what’s inside them. You won’t use this command often, but using it tofind out how a particular Java statement works is fun, sometimes. You can alsouse it to find out what methods are available for a class if you don’t have thesource code that was used to create the class.

Here is the general format:

javapfilename [options]

The following is typical of the information you getwhen you run the javap command:

C:\java\samples>javapHelloApp

Compiledfrom "HelloApp.java"

publicclass HelloApp extends java.lang.Object{

public HelloApp();

public static voidmain(java.lang.String[]);

}

As you can see, the javap command indicates that the HelloApp class was compiled from the HelloApp.java file and that it consists of a HelloApppublic class and a mainpublic method.

You may want to use two options with the javap command. If you use the -c option, the javap command displays the actual Java bytecodes createdby the compiler for the class. (Java bytecode is the executable programcompiled from your Java source file.)

And if you use the -verbose option, the bytecodes — plus a ton of otherfascinating information about the innards of the class — are displayed. Here’sthe -c output for aclass named HelloApp:

C:\java\samples>javapHelloApp -c

Compiledfrom "HelloApp.java"

publicclass HelloApp extends java.lang.Object{

publicHelloApp();

Code:

0:  aload_0

1:  invokespecial   #1; //Method

java/lang/Object."<init>":()V

4:  return

publicstatic void main(java.lang.String[]);

Code:

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

}