Java in a Nutshell, 5th Edition [Electronic resources] نسخه متنی

Synopsis

Description

jdb is a
debugger for Java classes. It is text-based, command-line-oriented,
and has a command syntax like that of the Unix

dbx or

gdb debuggers used
with C and C++ programs.

jdb is written in Java, so it runs within a
Java interpreter. When

jdb is invoked with the name of a Java class, it
starts another copy of the

java interpreter,
using any interpreter options specified on the command line. The new
interpreter is started with special options that enable it to
communicate with

jdb . The new interpreter loads
the specified class file and then stops and waits for debugging
commands before executing the first byte code.

jdb can also debug a program that is already
running in another Java interpreter. Doing so requires that special
options be passed to both the

java interpreter
and to

jdb . See the -attach
option below.

jdb expression syntax

jdb debugging
commands such as print, dump,
and suspend allow you to refer to classes,
objects, methods, fields, and threads in the program being debugged.
You can refer to classes by name, with or without their package
names. You can also refer to static class members
by name. You can refer to individual objects by object ID, which is
an eight-digit hexadecimal integer. Or, when the classes you are
debugging contain local variable information, you can often use local
variable names to refer to objects. You can use normal Java syntax to
refer to the fields of an object and the elements of an array; you
can also use this syntax to write quite complex expressions. As of
Java 1.3,

jdb even supports method invocation
using standard Java syntax.

Options

When
invoking

jdb with a specified class file, any of
the

java interpreter options can be specified.
See the

java reference page for an explanation
of these options. In addition,

jdb supports the
following options:

-attach [host:]port

Specifies that

jdb
should connect to the Java VM that is already running on the
specified host (or the local host, if unspecified) and listening for
debugging connections on the specified port. Java 1.3 and later.

In order to use

jdb to connect to a running VM
in this way, the VM must have been started with special command-line
options. In Java 1.3 and 1.4, use these options:

% java -Xdebug -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n

In Java 5.0, use these options
instead:

% java -agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n

The Java debugging architecture allows a complex set of
interpreter-to-debugger connection
options, and

java and

jdb
provide a complex set of options and suboptions to enable it. A
detailed description of those options is beyond the scope of this
book.

-connect connector:args

This option provides the most general and
flexible method for connecting

jdb to the
process to be debugged. Specify the name of a
connector (a Java class) followed by a
colon and a comma-separated list of arguments in name=value form.
Java 1.4 and later. See -listconnectors for
available connectors and their arguments.

-help

Displays a usage message listing supported options.

-launch

Starts the specified application when

jdb
starts. This avoids the need to explicitly use the
run command to start it. Java 1.3 and later.

-listconnectors

List available connection methods. Each connector is a Java class and
a list of arguments. Java 5.0 and later. See the
-connect option.

-listen port

Listens on the specified port for a Java
VM to connect to the debugger. To make this work, the VM must be with
options like these:

% java -agentlib:jdwp=transport=dt_socket,address=8000,server=n,suspend=y

Java 1.4 and later.

-listenany

Like the -listen option but

jdb picks a port to listen on and prints out the
port number for use when launching the Java process to debug. Java
1.4 and later.

-sourcepath path

Specifies the locations

jdb searches when attempting to find source
files that correspond to the class files being debugged. If
unspecified,

jdb uses the classpath by default.
Java 1.3 and later.

-tclient

Tells

jdb to invoke the client version of the
Java interpreter.

-tserver

Tells

jdb to invoke the server version of the
Java interpreter.

-version

Displays the

jdb version number and exits.

Commands

jdb
understands the following debugging commands. Use the
help command for more.

? or

help

Lists all supported commands, with a
short explanation of each.

!!

A shorthand command that is
replaced with the text of the last command entered. It can be
followed with additional text to append to that command.

catch [ exception-class ]

Causes a breakpoint whenever the
specified exception is thrown. If no exception is specified, the
command lists the exceptions currently being caught. Use
ignore to stop these breakpoints from occurring.

classes

Lists all classes that have been loaded.

clear

Lists all currently set breakpoints.

clear class.method [

( param-type...

) ]

Clears the breakpoint set in the specified method of the specified
class.

clear [ class:line ]

Removes the breakpoint set at the specified line of the specified
class.

cont

Resumes execution. This command should be used when the current
thread is stopped at a breakpoint.

down [ n ]

Moves down n frames in the call stack of
the current thread. If n is not specified,
moves down one frame.

dump id...

Prints the value of all fields of the specified object or objects. If
you specify the name of a class, dump displays all
class (static) methods and variables of the class and also displays
the superclass and list of implemented interfaces. Objects and
classes can be specified by name or by their eight-digit hexadecimal
ID numbers. Threads can also be specified with the shorthand
t@thread-number.

exit or

quit

Quits

jdb .

gc

Runs the garbage collector to force
unused objects to be reclaimed.

ignore exception-class

Does not treat the specified exception as a breakpoint. This command
turns off a catch command. This command does not
cause the Java interpreter to ignore exceptions; it merely tells

jdb to ignore them.

list [ line-number ]

Lists the specified line of source code as well as several lines that
appear before and after it. If no line number is specified, uses the
line number of the current stack frame of the current thread. The
lines listed are from the source file of the current stack frame of
the current thread. Use the use command to tell

jdb where to find source files.

list method

Displays the source code of the specified method.

load classname

Loads the specified class into

jdb .

locals

Displays a list of local variables for the current stack frame. Java
code must be compiled with the -g option in order
to contain local variable information.

methods class

Lists all methods of the specified
class. Use dump to list the instance variables of
an object or the class (static) variables of a class.

print id...

Prints the value of the specified item or items. Each item can be a
class, object, field, or local variable, and can be specified by name
or by eight-digit hexadecimal ID number. You can also refer to
threads with the special syntax
t@thread-number. The
print command displays an
object's value by invoking its
toString() method.

next

Executes the current line of source code, including any method calls
it makes. See also step.

resume [ thread-id... ]

Resumes execution of the specified thread or threads. If no threads
are specified, all suspended threads are resumed. See also
suspend.

run [ class ] [ args ]

Runs the main()
method of the specified class, passing the specified arguments to it.
If no class or arguments are specified, uses the class and arguments
specified on the

jdb command line.

step

Runs the current line of the current thread and stops again. If the
line invokes a method, steps into that method and stops. See also
next.

stepi

Executes a single Java VM instruction.

step up

Runs until the current method returns to its caller and stops again.

stop

Lists current breakpoints.

stop at class:line

Sets a breakpoint at the specified line of the specified class.
Program execution stops when it reaches this line. Use
clear to remove a breakpoint.

stop in class.method [

( param-type...

) ]

Sets a breakpoint at the beginning of the specified method of the
specified class. Program execution stops when it enters the method.
Use clear to remove a breakpoint.

suspend [ thread-id... ]

Suspends the specified thread or
threads. If no threads are specified, suspends all running threads.
Use resume to restart them.

thread thread-id

Sets the current thread to the specified thread number. This thread
is used implicitly by a number of other

jdb
commands.

threadgroup name

Sets the current thread group.

threadgroups

Lists all thread groups running in the Java interpreter session being
debugged.

threads [ threadgroup ]

Lists all threads in the named thread group. If no thread group is
specified, lists all threads in the current thread group (specified
by threadgroup).

up [ n ]

Moves up n frames in the call stack of the
current thread. If n is not specified, moves up
one frame.

use [ source-file-path ]

Sets the path used by

jdb to look up source files for the classes
being debugged. If no path is specified, displays the current source
path.

where [thread-id ] [

all ]

Displays a stack trace for the specified thread. If no thread is
specified, displays a stack trace for the current thread. If
all is specified, displays a stack trace for all
threads.

wherei [thread-id x]

Displays a stack
trace for the specified or current thread, including detailed program
counter information.

Environment

CLASSPATH

Specifies an ordered list
(colon-separated on Unix, semicolon-separated on Windows systems) of
directories, ZIP files, and JAR archives in which

jdb should look for class definitions. When a
path is specified with this environment variable,

jdb always implicitly appends the location of
the system classes to the end of the path. If this environment
variable is not specified, the default path is the current directory
and the system classes. This variable is overridden by the
-classpath option.

See also

java