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  22 .\"
  23 .\"     Arch: generic
  24 .\"     Software: JDK 8
  25 .\"     Date: 21 November 2013
  26 .\"     SectDesc: Basic Tools
  27 .\"     Title: jdb.1
  28 .\"
  29 .if n .pl 99999
  30 .TH jdb 1 "21 November 2013" "JDK 8" "Basic Tools"
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  51 .SH NAME    
  52 jdb \- Finds and fixes bugs in Java platform programs\&.
  53 .SH SYNOPSIS    
  54 .sp     
  55 .nf     
  57 \fBjdb\fR [\fIoptions\fR] [\fIclassname\fR]  [\fIarguments\fR]
  58 .fi     
  59 .sp     
  60 .TP     
  61 \fIoptions\fR
  62 Command-line options\&. See Options\&.
  63 .TP     
  64 \fIclass\fRname
  65 Name of the main class to debug\&.
  66 .TP     
  67 \fIarguments\fR
  68 Arguments passed to the \f3main()\fR method of the class\&.
  70 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
  72 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:
  73 .sp     
  74 .nf     
  75 \f3jdb MyClass\fP
  76 .fi     
  77 .nf     
  78 \f3\fP
  79 .fi     
  80 .sp     
  81 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\&.
  82 .PP
  83 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\&.
  84 .sp     
  85 .nf     
  86 \f3java \-agentlib:jdwp=transport=dt_socket,server=y,suspend=n MyClass\fP
  87 .fi     
  88 .nf     
  89 \f3\fP
  90 .fi     
  91 .sp     
  92 You can then attach the \f3jdb\fR command to the JVM with the following command:
  93 .sp     
  94 .nf     
  95 \f3jdb \-attach 8000\fP
  96 .fi     
  97 .nf     
  98 \f3\fP
  99 .fi     
 100 .sp     
 101 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\&.
 102 .PP
 103 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\&.
 105 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\&.
 106 .TP     
 107 help or ?
 108 The \f3help\fR or \f3?\fR commands display the list of recognized commands with a brief description\&.
 109 .TP     
 110 run
 111 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\&.
 112 .TP     
 113 cont
 114 Continues execution of the debugged application after a breakpoint, exception, or step\&.
 115 .TP     
 116 print
 117 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\&.
 119 \fINote:\fR To display local variables, the containing class must have been compiled with the \f3javac -g\fR option\&.
 121 The \f3print\fR command supports many simple Java expressions including those with method invocations, for example:
 122 .sp     
 123 .nf     
 124 \f3print MyClass\&.myStaticField\fP
 125 .fi     
 126 .nf     
 127 \f3print myObj\&.myInstanceField\fP
 128 .fi     
 129 .nf     
 130 \f3print i + j + k (i, j, k are primities and either fields or local variables)\fP
 131 .fi     
 132 .nf     
 133 \f3print myObj\&.myMethod() (if myMethod returns a non\-null)\fP
 134 .fi     
 135 .nf     
 136 \f3print new java\&.lang\&.String("Hello")\&.length()\fP
 137 .fi     
 138 .nf     
 139 \f3\fP
 140 .fi     
 141 .sp     
 143 .TP     
 144 dump
 145 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\&.
 146 .TP     
 147 threads
 148 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\&.
 149 .sp     
 150 .nf     
 151 \f34\&. (java\&.lang\&.Thread)0x1 main      running\fP
 152 .fi     
 153 .nf     
 154 \f3\fP
 155 .fi     
 156 .sp     
 158 .TP     
 159 thread
 160 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\&.
 161 .TP     
 162 where
 163 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\&.
 165 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\&.
 167 Breakpoints can be set in JDB at line numbers or at the first instruction of a method, for example:
 168 .TP 0.2i    
 169 \(bu
 170 The command \f3stop at MyClass:22\fR sets a breakpoint at the first instruction for line 22 of the source file containing \f3MyClass\fR\&.
 171 .TP 0.2i    
 172 \(bu
 173 The command \f3stop in java\&.lang\&.String\&.length\fR sets a breakpoint at the beginning of the method \f3java\&.lang\&.String\&.length\fR\&.
 174 .TP 0.2i    
 175 \(bu
 176 The command \f3stop in MyClass\&.<clinit>\fR uses \f3<clinit>\fR to identify the static initialization code for \f3MyClass\fR\&.
 177 .PP
 178 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\&.
 179 .PP
 180 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\&.
 181 .SS STEPPING    
 182 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\&.
 184 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\&.
 185 .PP
 186 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\&.
 187 .PP
 188 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\&.
 189 .SH OPTIONS    
 190 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\&.
 191 .PP
 192 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
 193 .TP
 194 -help
 195 .br
 196 Displays a help message\&.
 197 .TP
 198 -sourcepath \fIdir1:dir2: \&. \&. \&.\fR
 199 .br
 200 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 (\&.)\&.
 201 .TP
 202 -attach \fIaddress\fR
 203 .br
 204 Attaches the debugger to a running JVM with the default connection mechanism\&.
 205 .TP
 206 -listen \fIaddress\fR
 207 .br
 208 Waits for a running JVM to connect to the specified address with a standard connector\&.
 209 .TP
 210 -launch
 211 .br
 212 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\&.
 213 .TP
 214 -listconnectors
 215 .br
 216 List the connectors available in this JVM\&.
 217 .TP
 218 -connect connector-name:\fIname1=value1\fR
 219 .br
 220 Connects to the target JVM with the named connector and listed argument values\&.
 221 .TP
 222 -dbgtrace [\fIflags\fR]
 223 .br
 224 Prints information for debugging the \f3jdb\fR command\&.
 225 .TP
 226 -tclient
 227 .br
 228 Runs the application in the Java HotSpot VM client\&.
 229 .TP
 230 -tserver
 231 .br
 232 Runs the application in the Java HotSpot VM server\&.
 233 .TP
 234 -J\fIoption\fR
 235 .br
 236 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)\&.
 238 .TP
 239 -v -verbose[:\fIclass\fR|gc|jni]
 240 .br
 241 Turns on verbose mode\&.
 242 .TP
 243 -D\fIname\fR=\fIvalue\fR
 244 .br
 245 Sets a system property\&.
 246 .TP
 247 -classpath \fIdir\fR
 248 .br
 249 Lists directories separated by colons in which to look for classes\&.
 250 .TP
 251 -X\fIoption\fR
 252 .br
 253 Nonstandard target JVM option\&.
 254 .SH SEE\ ALSO    
 255 .TP 0.2i    
 256 \(bu
 257 javac(1)
 258 .TP 0.2i    
 259 \(bu
 260 java(1)
 261 .TP 0.2i    
 262 \(bu
 263 javap(1)
 264 .RE
 265 .br
 266 'pl 8.5i
 267 'bp