Logging

5 Logging

This topic describes the various logging options available in Oracle JRockit and HotSpot:

Verbose Logging

Verbose logging in HotSpot can be enabled using the -verbose option. There are some specific flags that can be used with this option to get area-specific verbose output.

The following table lists various logging options available in Oracle JRockit and compares them with the options available in HotSpot:

Table 5-1 Verbose Logging

Oracle JRockit Verbose Module	HotSpot Option	Notes
`alloc`	NA	NA
`class`	`-verbose:class`	Displays information about the classes that are being loaded.
`codegen`	NA	NA
`compaction`	NA	NA
`cpuinfo`	NA	NA
`exceptions`	NA	NA
`gc`	`-verbose:gc`	Displays information about each garbage collection (GC) event.
`gcheuristic`	NA	NA
`gcpause`	NA	NA
`gcpausetree`	NA	NA
`gcreport`	NA	NA
`load`	NA	NA
`memory`	NA	NA
`memdbg`	NA	NA
`opt`	NA	NA
`refobj`	NA	NA
`starttime`	NA	NA
`shutdown`	NA	NA
`systemgc`	NA	NA
`timing`	NA	NA
NA	`-verbose:jni`	Displays information about the use of native methods and other Java Native Interface (JNI) activity.

HotSpot Logging Options

These are some of the common logging options available in HotSpot that can be used to enable the diagnostic output for a specific subsystem within the HotSpot JVM.

Table 5-2 Logging Options

HotSpot Logging Options	Notes
`-Xlog`	Enables the common logging system for all JVM components.
`-Xloggc:<filename>`	Sets the file to which verbose GC event information must be redirected for logging. The information written to this file is similar to the output of `-verbose:gc` with the time elapsed from the first GC event preceding each logged event. The `-Xloggc` option overrides the `-verbose:gc`, if both are given with the same `java` command. Note: This option was deprecated in JDK 9.
`-XX:LogFile=<path>`	Sets the path and file name where the log data is written.
`-XX:+LogCompilation`	Enables logging of compilation activity to a file named `hotspot.log` in the current working directory. You can specify a different log file path and name using the `-XX:LogFile` option. The `-XX:+LogCompilation` option must be used together with the `-XX:UnlockDiagnosticVMOptions` option that unlocks diagnostic JVM options.
`-XX:+PrintCommandLineFlags`	Enables printing of the selected JVM flags that appeared on the command-line.
`-XX:+PrintGC`	Enables printing of messages at every GC. Note: This option was deprecated in JDK 9.
`-XX:+PrintGCDetails`	Enables printing of detailed messages at every GC. Note: This option was deprecated in JDK 9.
`-XX:+PrintNMTStatistics`	Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled.
`-XX:+PrintNMTStatistics`	Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled.
`-XX:+PrintAssembly`	Enables printing of assembly code resulting from JIT compilation of Java bytecode by using the external `disassembler.so` library. This option enables you to view the generated code, which helps you to diagnose the performance issues. This option must be used together with the `-XX:UnlockDiagnosticVMOptions` option that unlocks diagnostic JVM options.
`-XX:+PrintCompilation`	Enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled.
`-XX:+PrintInlining`	Enables printing of inlining decisions. This option enables you to view the methods that are getting inlined.
`-XX:+PrintClassHistogram`	Enables printing of a class instance histogram after a Control+C event (SIGTERM). By default, this option is disabled.
`-XX:+PrintConcurrentLocks`	Enables printing of `java.util.concurrent` locks after a Control+C event (SIGTERM). By default, this option is disabled.

Table 5-3 GC Logging Options

GC Logging Options	Notes
`-Xlog:gc`	Enables printing of messages at every GC. The `gc` is the main tag to log all GC related information. The `gc` tag is combined with other tags to log specific information. A few tags are listed in the following rows.
	`-Xlog:gc+region=trace`	Enables the printing of information about the regions that are allocated and that are reclaimed by the G1 collector.
	`-Xlog:gc+ergo*=trace`	Enables printing of information about adaptive generation sizing.
	`-Xlog:safepoint`	Enables printing of the time elapsed from the last pause (for example, a GC pause).
	`-Xlog:gc+task=trace`	Enables printing of time stamps for every individual GC worker thread task.
	`-Xlog:gc::uptime,tid`	Enables printing of time stamps at every GC using the decorators `uptime` and `tid`.
	`-Xlog:gc+stringdedup`	Prints detailed deduplication statistics.
	`-Xlog:gc+age=trace`	Enables printing of tenuring age information.
	`-Xlog:gc*`	Enables printing of detailed messages at every GC.
`-Xlog:gc:file=<filename>`	Logs messages with the `gc` tag to the file name specified. For example the option `-Xlog:gc:file=gc.txt` logs the messages to the `gc.txt` file.

See Enable Logging with the JVM Unified Logging Framework in the Java Platform, Standard Edition Tools Reference guide.