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| .\" Arch: generic |
| .\" Software: JDK 8 |
| .\" Date: 21 November 2013 |
| .\" SectDesc: Basic Tools |
| .\" Title: jdb.1 |
| .\" |
| .if n .pl 99999 |
| .TH jdb 1 "21 November 2013" "JDK 8" "Basic Tools" |
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| .\" * MAIN CONTENT STARTS HERE * |
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| |
| .SH NAME |
| jdb \- Finds and fixes bugs in Java platform programs\&. |
| .SH SYNOPSIS |
| .sp |
| .nf |
| |
| \fBjdb\fR [\fIoptions\fR] [\fIclassname\fR] [\fIarguments\fR] |
| .fi |
| .sp |
| .TP |
| \fIoptions\fR |
| Command-line options\&. See Options\&. |
| .TP |
| \fIclass\fRname |
| Name of the main class to debug\&. |
| .TP |
| \fIarguments\fR |
| Arguments passed to the \f3main()\fR method of the class\&. |
| .SH DESCRIPTION |
| The Java Debugger (JDB) is a simple command-line debugger for Java classes\&. The \f3jdb\fR command and its options call the JDB\&. The \f3jdb\fR 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 |
| .SS 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 \f3jdb\fR command for the \f3java\fR command in the command line\&. For example, if your application\&'s main class is \f3MyClass\fR, then use the following command to debug it under JDB: |
| .sp |
| .nf |
| \f3jdb MyClass\fP |
| .fi |
| .nf |
| \f3\fP |
| .fi |
| .sp |
| When started this way, the \f3jdb\fR command calls a second JVM with the specified parameters, loads the specified class, and stops the JVM before executing that class\&'s first instruction\&. |
| .PP |
| Another way to use the \f3jdb\fR command is by attaching it to a JVM that is already running\&. Syntax for starting a JVM to which the \f3jdb\fR 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\&. |
| .sp |
| .nf |
| \f3java \-agentlib:jdwp=transport=dt_socket,server=y,suspend=n MyClass\fP |
| .fi |
| .nf |
| \f3\fP |
| .fi |
| .sp |
| You can then attach the \f3jdb\fR command to the JVM with the following command: |
| .sp |
| .nf |
| \f3jdb \-attach 8000\fP |
| .fi |
| .nf |
| \f3\fP |
| .fi |
| .sp |
| The \f3MyClass\fR argument is not specified in the \f3jdb\fR command line in this case because the \f3jdb\fR command is connecting to an existing JVM instead of launching a new JVM\&. |
| .PP |
| There are many other ways to connect the debugger to a JVM, and all of them are supported by the \f3jdb\fR command\&. The Java Platform Debugger Architecture has additional documentation on these connection options\&. |
| .SS BASIC\ JDB\ COMMANDS |
| The following is a list of the basic \f3jdb\fR commands\&. The JDB supports other commands that you can list with the \f3-help\fR option\&. |
| .TP |
| help or ? |
| The \f3help\fR or \f3?\fR commands display the list of recognized commands with a brief description\&. |
| .TP |
| run |
| After you start JDB and set breakpoints, you can use the \f3run\fR command to execute the debugged application\&. The \f3run\fR command is available only when the \f3jdb\fR command starts the debugged application as opposed to attaching to an existing JVM\&. |
| .TP |
| cont |
| Continues execution of the debugged application after a breakpoint, exception, or step\&. |
| .TP |
| 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\&. |
| |
| \fINote:\fR To display local variables, the containing class must have been compiled with the \f3javac -g\fR option\&. |
| |
| The \f3print\fR command supports many simple Java expressions including those with method invocations, for example: |
| .sp |
| .nf |
| \f3print MyClass\&.myStaticField\fP |
| .fi |
| .nf |
| \f3print myObj\&.myInstanceField\fP |
| .fi |
| .nf |
| \f3print i + j + k (i, j, k are primities and either fields or local variables)\fP |
| .fi |
| .nf |
| \f3print myObj\&.myMethod() (if myMethod returns a non\-null)\fP |
| .fi |
| .nf |
| \f3print new java\&.lang\&.String("Hello")\&.length()\fP |
| .fi |
| .nf |
| \f3\fP |
| .fi |
| .sp |
| |
| .TP |
| dump |
| For primitive values, the \f3dump\fR command is identical to the \f3print\fR command\&. For objects, the \f3dump\fR command prints the current value of each field defined in the object\&. Static and instance fields are included\&. The \f3dump\fR command supports the same set of expressions as the \f3print\fR command\&. |
| .TP |
| 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 \f3java\&.lang\&.Thread\fR, the thread name is \f3main\fR, and it is currently running\&. |
| .sp |
| .nf |
| \f34\&. (java\&.lang\&.Thread)0x1 main running\fP |
| .fi |
| .nf |
| \f3\fP |
| .fi |
| .sp |
| |
| .TP |
| thread |
| Select a thread to be the current thread\&. Many \f3jdb\fR commands are based on the setting of the current thread\&. The thread is specified with the thread index described in the threads command\&. |
| .TP |
| where |
| The \f3where\fR command with no arguments dumps the stack of the current thread\&. The \f3where\fR\f3all\fR command dumps the stack of all threads in the current thread group\&. The \f3where\fR\f3threadindex\fR 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 \f3suspend\fR command, then local variables and fields can be displayed with the \f3print\fR and \f3dump\fR commands\&. The \f3up\fR and \f3down\fR commands select which stack frame is the current stack frame\&. |
| .SS BREAKPOINTS |
| Breakpoints can be set in JDB at line numbers or at the first instruction of a method, for example: |
| .TP 0.2i |
| \(bu |
| The command \f3stop at MyClass:22\fR sets a breakpoint at the first instruction for line 22 of the source file containing \f3MyClass\fR\&. |
| .TP 0.2i |
| \(bu |
| The command \f3stop in java\&.lang\&.String\&.length\fR sets a breakpoint at the beginning of the method \f3java\&.lang\&.String\&.length\fR\&. |
| .TP 0.2i |
| \(bu |
| The command \f3stop in MyClass\&.<clinit>\fR uses \f3<clinit>\fR to identify the static initialization code for \f3MyClass\fR\&. |
| .PP |
| 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, \f3MyClass\&.myMethod(int,java\&.lang\&.String)\fR or \f3MyClass\&.myMethod()\fR\&. |
| .PP |
| The \f3clear\fR command removes breakpoints using the following syntax: \f3clear MyClass:45\fR\&. Using the \f3clear\fR or \f3stop\fR command with no argument displays a list of all breakpoints currently set\&. The \f3cont\fR command continues execution\&. |
| .SS STEPPING |
| The \f3step\fR command advances execution to the next line whether it is in the current stack frame or a called method\&. The \f3next\fR command advances execution to the next line in the current stack frame\&. |
| .SS EXCEPTIONS |
| When an exception occurs for which there is not a \f3catch\fR 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 \f3jdb\fR command to diagnose the cause of the exception\&. |
| .PP |
| Use the \f3catch\fR command to cause the debugged application to stop at other thrown exceptions, for example: \f3catch java\&.io\&.FileNotFoundException\fR or \f3catch\fR\f3mypackage\&.BigTroubleException\fR\&. Any exception that is an instance of the specified class or subclass stops the application at the point where it is thrown\&. |
| .PP |
| The \f3ignore\fR command negates the effect of an earlier \f3catch\fR command\&. The \f3ignore\fR command does not cause the debugged JVM to ignore specific exceptions, but only to ignore the debugger\&. |
| .SH OPTIONS |
| When you use the \f3jdb\fR command instead of the \f3java\fR command on the command line, the \f3jdb\fR command accepts many of the same options as the \f3java\fR command, including \f3-D\fR, \f3-classpath\fR, and \f3-X\fR options\&. The following list contains additional options that are accepted by the \f3jdb\fR command\&. |
| .PP |
| 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 |
| .TP |
| -help |
| .br |
| Displays a help message\&. |
| .TP |
| -sourcepath \fIdir1:dir2: \&. \&. \&.\fR |
| .br |
| 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 (\&.)\&. |
| .TP |
| -attach \fIaddress\fR |
| .br |
| Attaches the debugger to a running JVM with the default connection mechanism\&. |
| .TP |
| -listen \fIaddress\fR |
| .br |
| Waits for a running JVM to connect to the specified address with a standard connector\&. |
| .TP |
| -launch |
| .br |
| Starts the debugged application immediately upon startup of JDB\&. The \f3-launch\fR option removes the need for the \f3run\fR 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 \f3cont\fR command to continue execution\&. |
| .TP |
| -listconnectors |
| .br |
| List the connectors available in this JVM\&. |
| .TP |
| -connect connector-name:\fIname1=value1\fR |
| .br |
| Connects to the target JVM with the named connector and listed argument values\&. |
| .TP |
| -dbgtrace [\fIflags\fR] |
| .br |
| Prints information for debugging the \f3jdb\fR command\&. |
| .TP |
| -tclient |
| .br |
| Runs the application in the Java HotSpot VM client\&. |
| .TP |
| -tserver |
| .br |
| Runs the application in the Java HotSpot VM server\&. |
| .TP |
| -J\fIoption\fR |
| .br |
| Passes \f3option\fR to the JVM, where option is one of the options described on the reference page for the Java application launcher\&. For example, \f3-J-Xms48m\fR sets the startup memory to 48 MB\&. See java(1)\&. |
| .SH OPTIONS\ FORWARDED\ TO\ THE\ DEBUGGER\ PROCESS |
| .TP |
| -v -verbose[:\fIclass\fR|gc|jni] |
| .br |
| Turns on verbose mode\&. |
| .TP |
| -D\fIname\fR=\fIvalue\fR |
| .br |
| Sets a system property\&. |
| .TP |
| -classpath \fIdir\fR |
| .br |
| Lists directories separated by colons in which to look for classes\&. |
| .TP |
| -X\fIoption\fR |
| .br |
| Nonstandard target JVM option\&. |
| .SH SEE\ ALSO |
| .TP 0.2i |
| \(bu |
| javac(1) |
| .TP 0.2i |
| \(bu |
| java(1) |
| .TP 0.2i |
| \(bu |
| javah(1) |
| .TP 0.2i |
| \(bu |
| javap(1) |
| .RE |
| .br |
| 'pl 8.5i |
| 'bp |
