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How do I redirect verbose garbage collection output to a file? Sun’s website shows an example for Unix but it doesn't work for Windows.

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4 Answers 4

Reset to default
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From the output of java -X:

    -Xloggc:<file>    log GC status to a file with time stamps

Documented here:

-Xloggc:filename

Sets the file to which verbose GC events information should be redirected for logging. The information written to this file is similar to the output of -verbose:gc with the time elapsed since the first GC event preceding each logged event. The -Xloggc option overrides -verbose:gc if both are given with the same java command.

Example:

    -Xloggc:garbage-collection.log

So the output looks something like this:

0.590: [GC 896K->278K(5056K), 0.0096650 secs]
0.906: [GC 1174K->774K(5056K), 0.0106856 secs]
1.320: [GC 1670K->1009K(5056K), 0.0101132 secs]
1.459: [GC 1902K->1055K(5056K), 0.0030196 secs]
1.600: [GC 1951K->1161K(5056K), 0.0032375 secs]
1.686: [GC 1805K->1238K(5056K), 0.0034732 secs]
1.690: [Full GC 1238K->1238K(5056K), 0.0631661 secs]
1.874: [GC 62133K->61257K(65060K), 0.0014464 secs]
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java --Xloggc:firstgen.log -Xloggc:secondgen.log doesn't seem to work by the way.
Also, this doesnt increase the output, as I expected. There are no additional details after using these arguments: -Xloggc:gc.log -XX:-PrintGCDetails
"-XX:+PrintGCDetails" seems to work with -Xloggc. I don't know why the Sun page has a - instead of a +.
that worked. thanks! now, i use these options: set JAVA_OPTS=%JAVA_OPTS% -Xloggc:logs\gc.log -XX:+PrintGCDetails -XX:MaxPermSize=128m
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If in addition you want to pipe the output to a separate file, you can do:

On a Sun JVM:

-Xloggc:C:\whereever\jvm.log -verbose:gc -XX:+PrintGCDateStamps

ON an IBM JVM:

-Xverbosegclog:C:\whereever\jvm.log
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If you use -Xloggc, you don't need -verbose:gc.
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Java 9 & Unified JVM Logging

JEP 158 introduces a common logging system for all components of the JVM which will change (and IMO simplify) how logging works with GC. JEP 158 added a new command-line option to control logging from all components of the JVM: 

-Xlog

For example, the following option:

-Xlog:gc

will log messages tagged with gc tag using info level to stdout. Or this one:

-Xlog:gc=debug:file=gc.txt:none

would log messages tagged with gc tag using debug level to a file called gc.txt with no decorations. For more detailed discussion, you can checkout the examples in the JEP page.

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With JDK 16 you even get a warning if you use the old -Xloggc instead of -Xlog:gc:...
How can I append the log to an existing file?
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To add to the above answers, there's a good article: Useful JVM Flags – Part 8 (GC Logging) by Patrick Peschlow.

A brief excerpt:

The flag -XX:+PrintGC (or the alias -verbose:gc) activates the “simple” GC logging mode

By default the GC log is written to stdout. With -Xloggc:<file> we may instead specify an output file. Note that this flag implicitly sets -XX:+PrintGC and -XX:+PrintGCTimeStamps as well.

If we use -XX:+PrintGCDetails instead of -XX:+PrintGC, we activate the “detailed” GC logging mode which differs depending on the GC algorithm used.

With -XX:+PrintGCTimeStamps a timestamp reflecting the real time passed in seconds since JVM start is added to every line.

If we specify -XX:+PrintGCDateStamps each line starts with the absolute date and time.

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