jdb Man page

jdb Basic Tools jdb

NAME

jdb – Finds and fixes bugs in Java platform programs.

SYNOPSIS

jdb [options] [classname] [arguments]

options
Command-line options. See Options.

classname
Name of the main class to debug.

arguments
Arguments passed to the main() method of the class.

DESCRIPTION

The Java Debugger (JDB) is a simple command-line debugger for Java
classes. The jdb command and its options call the JDB. The jdb command
demonstrates the Java Platform Debugger Architecture (JDBA) and
provides inspection and debugging of a local or remote Java Virtual
Machine (JVM). See Java Platform Debugger Architecture (JDBA) at
http://docs.oracle.com/javase/8/docs/technotes/guides/jpda/index.html

START A JDB SESSION
There are many ways to start a JDB session. The most frequently used
way is to have JDB launch a new JVM with the main class of the
application to be debugged. Do this by substituting the jdb command for
the java command in the command line. For example, if your
application’s main class is MyClass, then use the following command to
debug it under JDB:

jdb MyClass

When started this way, the jdb command calls a second JVM with the
specified parameters, loads the specified class, and stops the JVM
before executing that class’s first instruction.

Another way to use the jdb command is by attaching it to a JVM that is
already running. Syntax for starting a JVM to which the jdb command
attaches when the JVM is running is as follows. This loads in-process
debugging libraries and specifies the kind of connection to be made.

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n MyClass

You can then attach the jdb command to the JVM with the following
command:

jdb -attach 8000

The MyClass argument is not specified in the jdb command line in this
case because the jdb command is connecting to an existing JVM instead
of launching a new JVM.

There are many other ways to connect the debugger to a JVM, and all of
them are supported by the jdb command. The Java Platform Debugger
Architecture has additional documentation on these connection options.

BASIC JDB COMMANDS
The following is a list of the basic jdb commands. The JDB supports
other commands that you can list with the -help option.

help or ?
The help or ? commands display the list of recognized commands
with a brief description.

run After you start JDB and set breakpoints, you can use the run
command to execute the debugged application. The run command is
available only when the jdb command starts the debugged
application as opposed to attaching to an existing JVM.

cont Continues execution of the debugged application after a
breakpoint, exception, or step.

print Displays Java objects and primitive values. For variables or
fields of primitive types, the actual value is printed. For
objects, a short description is printed. See the dump command to
find out how to get more information about an object.

Note: To display local variables, the containing class must have
been compiled with the javac -g option.

The print command supports many simple Java expressions
including those with method invocations, for example:

print MyClass.myStaticField
print myObj.myInstanceField
print i + j + k (i, j, k are primities and either fields or local variables)
print myObj.myMethod() (if myMethod returns a non-null)
print new java.lang.String(“Hello”).length()

dump For primitive values, the dump command is identical to the print
command. For objects, the dump command prints the current value
of each field defined in the object. Static and instance fields
are included. The dump command supports the same set of
expressions as the print command.

threads
List the threads that are currently running. For each thread,
its name and current status are printed and an index that can be
used in other commands. In this example, the thread index is 4,
the thread is an instance of java.lang.Thread, the thread name
is main, and it is currently running.

4. (java.lang.Thread)0x1 main running

thread Select a thread to be the current thread. Many jdb commands are
based on the setting of the current thread. The thread is
specified with the thread index described in the threads
command.

where The where command with no arguments dumps the stack of the
current thread. The whereall command dumps the stack of all
threads in the current thread group. The wherethreadindex
command dumps the stack of the specified thread.

If the current thread is suspended either through an event such
as a breakpoint or through the suspend command, then local
variables and fields can be displayed with the print and dump
commands. The up and down commands select which stack frame is
the current stack frame.

BREAKPOINTS
Breakpoints can be set in JDB at line numbers or at the first
instruction of a method, for example:

· The command stop at MyClass:22 sets a breakpoint at the first
instruction for line 22 of the source file containing MyClass.

· The command stop in java.lang.String.length sets a breakpoint at the
beginning of the method java.lang.String.length.

· The command stop in MyClass. uses to identify the
static initialization code for MyClass.

When a method is overloaded, you must also specify its argument types
so that the proper method can be selected for a breakpoint. For
example, MyClass.myMethod(int,java.lang.String) or MyClass.myMethod().

The clear command removes breakpoints using the following syntax: clear
MyClass:45. Using the clear or stop command with no argument displays a
list of all breakpoints currently set. The cont command continues
execution.

STEPPING
The step command advances execution to the next line whether it is in
the current stack frame or a called method. The next command advances
execution to the next line in the current stack frame.

EXCEPTIONS
When an exception occurs for which there is not a catch statement
anywhere in the throwing thread’s call stack, the JVM typically prints
an exception trace and exits. When running under JDB, however, control
returns to JDB at the offending throw. You can then use the jdb command
to diagnose the cause of the exception.

Use the catch command to cause the debugged application to stop at
other thrown exceptions, for example: catch
java.io.FileNotFoundException or catchmypackage.BigTroubleException.
Any exception that is an instance of the specified class or subclass
stops the application at the point where it is thrown.

The ignore command negates the effect of an earlier catch command. The
ignore command does not cause the debugged JVM to ignore specific
exceptions, but only to ignore the debugger.

OPTIONS

When you use the jdb command instead of the java command on the command
line, the jdb command accepts many of the same options as the java
command, including -D, -classpath, and -X options. The following list
contains additional options that are accepted by the jdb command.

Other options are supported to provide alternate mechanisms for
connecting the debugger to the JVM it is to debug. For additional
documentation about these connection alternatives, see Java Platform
Debugger Architecture (JPDA) at
http://docs.oracle.com/javase/8/docs/technotes/guides/jpda/index.html

-help
Displays a help message.

-sourcepath dir1:dir2: . . .
Uses the specified path to search for source files in the
specified path. If this option is not specified, then use the
default path of dot (.).

-attach address
Attaches the debugger to a running JVM with the default
connection mechanism.

-listen address
Waits for a running JVM to connect to the specified address with
a standard connector.

-launch
Starts the debugged application immediately upon startup of JDB.
The -launch option removes the need for the run command. The
debugged application is launched and then stopped just before
the initial application class is loaded. At that point, you can
set any necessary breakpoints and use the cont command to
continue execution.

-listconnectors
List the connectors available in this JVM.

-connect connector-name:name1=value1
Connects to the target JVM with the named connector and listed
argument values.

-dbgtrace [flags] Prints information for debugging the jdb command.

-tclient
Runs the application in the Java HotSpot VM client.

-tserver
Runs the application in the Java HotSpot VM server.

-Joption
Passes option to the JVM, where option is one of the options
described on the reference page for the Java application
launcher. For example, -J-Xms48m sets the startup memory to 48
MB. See java.

OPTIONS FORWARDED TO THE DEBUGGER PROCESS
-v -verbose[:class|gc|jni] Turns on verbose mode.

-Dname=value
Sets a system property.

-classpath dir
Lists directories separated by colons in which to look for
classes.

-Xoption
Nonstandard target JVM option.

SEE ALSO

· javac

· java

· javah

· javap

JDK 8 21 November 2013 jdb