Debug with jdb

原文住所: http://www.javaworld.com/article/2077445/testing-debugging/debug-with-jdb.html
Q: How do you use jdb (included in the JDK 1.2 package) effectively to debug Java programs?
I've tried many times, but I am successful only in loading a class file to  jdb ; I can't debug it. The  help  command isn't much use.
A: You ask an interesting question. To be honest, I've neverused  jdb . I have always used the debugger provided by my IDE environment. So to answer your question I had to do a little research of my own.
It turns out that Sun considers  jdb  a proof of concept for the Java Debugger API. The Java Debugger API allows us to actually peek into the runtime and debug our code. The  jdb  is just one implementation of a debugger that uses the API. Compared to the visual debuggers with which I'm familiar (yes, I guess I'm a wimp), it's not the easiest debugger to use -- though it is similar to other command-line debuggers, such as gdb .
Anyhow, on to your question. Before attempting to debug your code, be sure to use the  -g  option while compiling your classes. This option tells the compiler to include debugging information in your class file.
Let's define a contrived class for testing:

publicclassTestMe{privateint int_value;privateString string_value;publicstaticvoid main(String[] args){TestMe testMe =newTestMe();

    testMe.setInt_value(1);

    testMe.setString_value("test");int integer = testMe.getInt_value();Stringstring= testMe.getString_value();String toString = testMe.toString();}publicTestMe(){}publicint getInt_value(){return int_value;}publicString getString_value(){return string_value;}publicvoid setInt_value(int value){

    int_value = value;}publicvoid setString_value(String value){

    string_value = value;}publicString toString(){return"String value: "+ string_value +" int value: "+ int_value;}}

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Start the debugger:

> jdb TestMe

You should see:

>Initializing jdb...>0xaa:class

Let's take a look at some basic commands. In order to set breakpoints, we need to know the line numbers or the method names of the places where we would like to break. To obtain a list of methods, simply use the  methods  command:

> methods TestMevoid main(java.lang.String[])void()int getInt_value()

   java.lang.String getString_value()void setInt_value(int)void setString_value(java.lang.String)

   java.lang.String toString()

Setting a breakpoint is simple. Use the following syntax:

stop in.[<argument_type,...>]

Or:

stop at :

We should start debugging at the beginning of the main method:

> stop inTestMe.main

   Breakpointsetin javaworld.TestMe.main

Now that we have a breakpoint, we can begin execution. To run up to the breakpoint, simply use the  run  command:

> run

run javaworld.TestMe

running ...

main[1]Breakpoint hit: javaworld.TestMe.main (TestMe:10)

At this point, the debugger halts execution at the first line of the main method. Notice that the cursor has changed to reflect the method that we are currently in.

The  list  command will display the code at the breakpoint. An arrow indicates the spot where the debugger has halted execution.

main[1] list

6privateString string_value;78publicstaticvoid main(String[] args)9{10=>TestMe testMe =newTestMe();11             testMe.setInt_value(1);12             testMe.setString_value("test");1314int integer = testMe.getInt_value();

main[1]

Next, we'll want to  step  through a few lines of code and see what's changed:

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.(TestMe:20)

main[1] locals

Method arguments:Local variables:this=String value:nullint value:0

main[1] list

1617String toString = testMe.toString();18}1920=>publicTestMe()21{22}2324publicint getInt_value()

main[1] step

main[1]Breakpoint hit: java.lang.Object.(Object:27)

main[1] list

Unable to find Object.java

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.(TestMe:22)

main[1] list

18}1920publicTestMe()21{22=>}2324publicint getInt_value()25{26return int_value;

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.main (TestMe:10)

main[1] list

6privateString string_value;78publicstaticvoid main(String[] args)9{10=>TestMe testMe =newTestMe();11             testMe.setInt_value(1);12             testMe.setString_value("test");1314int integer = testMe.getInt_value();

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.main (TestMe:11)

main[1] list

78publicstaticvoid main(String[] args)9{10TestMe testMe =newTestMe();11=>     testMe.setInt_value(1);12             testMe.setString_value("test");1314int integer = testMe.getInt_value();15Stringstring= testMe.getString_value();

main[1] locals

Method arguments:Local variables:

  args =

  testMe =String value:nullint value:0

After each  step , I called the  list  command to see where I was in the code. The return value from the command listed the line number, but somehow that didn't really help me very much.

As we  step , we see that the main method is constructing a  TestMe  instance. Each step takes us through the constructor and finally back into the main method. The  locals command lists all of the local variables visible in the current stack. We see that at this point in the main method there are only two local variables:  args  and  testMe .

By using  step , we can get inside any of the methods to see what is going on. When we combine  step  with the  locals  command we can see our variables:

main[1] step

main[1]Breakpoint hit: javaworld.TestMe.setInt_value (TestMe:36)

main[1] list

32}3334publicvoid setInt_value(int value)35{36=>     int_value = value;37}3839publicvoid setString_value(String value)40{

main[1] locals

Method arguments:Local variables:

  value =1this=String value:nullint value:0

If we  step  one more time, we end up in the  setInt_value() method. If we  step  two more times, the method will set the  int_value  member to  1  and return. (To check to see that the method set the value, use the  locals  command.)

Of course, when we  step , we won't always want to trace into each method we encounter. Some method calls can nest very deeply. If we were forced to trace through an entire hierarchy, we might never finish. Luckily,  jdb  has a way to execute a method without tracing into that method: the  next  command.

jdb  also provides a few other  step  commands. The  stepi  command executes the current instruction. In other words, the code at the  =>  will execute but the current line will not advance to the next instruction. You can call  stepi  a million times, but the  =>  displayed from the  list  command will not move.

jdb  also provides the  step up  command. The  step up  call executes until the current method returns to its caller. Simply put, this stepper executes a method and nothing else. Take the following code segment as an example:

int integer = testMe.getInt_value();

If this is our current line and we run  step up , the  getInt_value()  method will execute. However, that's all that will happen. The return value will not get set to  integer .

jdb  also allows us to set multiple breakpoints. To go from one breakpoint directly to the next,  jdb  provides the  cont  command.

Finally, there are times when we want to look at all the members of an instance or class. Luckily,  jdb  provides the  dump  and  print  commands:

main[1]dumpTestMeTestMe=0xa9:class(javaworld.TestMe){

    superclass =0x2:class(java.lang.Object)

    loader =(sun.misc.Launcher$AppClassLoader)0xaa}

main[1]printTestMeTestMe=0xa9:class(javaworld.TestMe)

main[1]dump testMe

testMe =(javaworld.TestMe)0xec{private java.lang.String string_value = test

    privateint int_value =1}

main[1]print testMe

testMe =String value: test int value:1