Tracing 
The VM does a lot of tracing, which can aid runtime analysis of Java programs. Examples are garbage collection statistics, method tracing or exception tracing.
Procedure
The verbose tracing for a certain VM component is controlled using the command line option -Xtrace in the form -Xtrace<traceflag>. Using just the option –Xtrace gives an overview of the existing trace flags.
Example
-XtraceMethods switches on the method traces.
-Xtrace* switches on all trace flags starting with a certain name.
Example: -XtraceClass* switches on the following trace flags: ClassLoading, ClassUnloading, ClassStatistic
Example: -Xtrace* switches on all trace flags.
-XtraceInfoHeader=<info header format>
Each trace line is preceded by a header. This header, which is similar to a prompt line on a UNIX shell, can be freely formatted and can contain a number of useful information.
Example: -XtraceInfoHeader=HELLO> causes the traces to look like this: HELLO> trace output.
-XtraceWarningHeader=<info header format>
-XtraceErrorHeader=<info header format>
Same as –XtraceInfoHeader for warning and error messages.
Placeholders can be used to embed information in the trace line. The following placeholders are available:
Placeholder |
Description |
|---|---|
@PID |
Process id |
@TID |
Thread id |
@SRC |
Source file and line of the C or Java code that caused the trace output |
@TIM @TIME |
Current time |
@DAT @DATE |
Current date |
@CMP |
A short marker for the component, which caused the trace, for example, “MTH” for method traces |
@VMID |
Unique VM ID |
Example
The command -XtraceInfoHeader=Date: @DAT(@TIM) Process:@PID Thread:@TID>> causes the traces to look like this:
Date: 2009-01-16(15:12:48) Number:0 Process:5832 Thread:5508 >> trace output...
Date: 2009-01-16(15:12:48) Number:1 Process:5832 Thread:5508 >> trace output...
Date: 2009-01-16(15:12:52) Number:2 Process:5832 Thread:5508 >> trace output...
—XtraceWarningHeader=<info header format>
-XtraceErrorHeader=<info header format>
Same as –XtraceInfoHeader for warning- and error messages.
Below is a summary of available trace flags. As mentioned before, just use the –Xtrace command line option alone to get the full list of flags.
Trace flag syntax |
Description |
|---|---|
-XtraceFile=<file> |
Write traces to the given output file. If no file is given, output will be sent to the console. Within the NetWeaver AS Java, you will find this trace output in dev_server0 and std_server0.out in the work directory |
-XtraceInfoHeader=<format> |
Set the header format for info messages |
-XtraceWarningHeader=<format> |
Set the header format for warning messages |
-XtraceErrorHeader=<format> |
Set the header format for error messages |
-XtraceHeader=<format> |
Set the header format for all messages |
-XtraceMethods |
Traces each method call |
-XtraceMethodsWithParameters |
Traces method calls and prints their parameters |
-XtraceExceptions |
Traces Java exception handling |
-XtraceClassLoading |
Traces class loading |
-XtraceClassUnloading |
Traces class unloading |
-XtraceJavaZipOpenClose |
Traces open/close operations of zip files from Java |
-XtraceJavaZipGetEntry |
Traces getEntry operations of zip files from Java |
-XtraceFileIoBasic |
Traces basic file IO (open/close) |
-XtraceFileIoBasic |
Traces basic file IO (open/close) |
-XtraceFileIoReadWrite |
Traces file IO read and write |
-XtraceNetworkIoBasic |
Traces basic network operations (bind, accept, listen, ...) |
-XtraceNetworkIoSendReceive |
Traces send and receive network operations |
-XtraceClassStatistic |
Traces class statistics after full GCs |
-XtraceDetailedClassStatistic |
Traces detailed class statistics after full GCs |
-XtraceGCStatisticLevel1 |
Basic GC statistic |
-XtraceGCStatisticLevel2 |
Detailed GC statistic |
-XtraceGCStatisticLevel3 |
Detailed GC statistic with basic information about the GC events |
-XtraceGCStatisticLevel4 |
Detailed GC statistic with detailed information about the GC events |
-XtraceGCStatisticLevel5 |
Detailed GC statistic with detailed information about the GC events and basic reference handling information |
-XtraceGCStatisticLevel6 |
Detailed GC statistic with detailed information about the GC events and detailed reference handling information |
-XtraceJdkInternals |
Traces JDK internals |
-XtraceOsDllLoad |
Traces resolution/loads of shared objects |
Setting trace options in a running system
In addition to the command line options, tracing settings can be changed on the fly at any time using the jvmmon tool. This allows enabling or disabling any available VM trace, therefore offering a facility to inspect problems when they occur.