brief introduction

In java programs, we use logs to locate and discover potential problems in our projects.In modern java projects, we use logging frameworks such as log4j or slf4j, Logback, and so on to handle logging issues.

JVM is the basis of running java programs. Events in JVM such as GC, class loading, JPMS, heap, threads and so on can actually be logged.Through these logs, we can monitor events in the JVM and, in turn, tune java applications.

The Xlog log log service introduced in JDK9 was created for this purpose.

Through xlog, JDK unifies all kinds of events in JVM and outputs them out in a unified form.Subsystems are distinguished by tag parameters, emergency events are distinguished by log level, and output addresses are configured by logging output.

Use of xlog

Let's start with the simplest example of using xlog:

java -Xlog -version

Output results:

[0.016s][info][os] Use of CLOCK_MONOTONIC is supported
[0.016s][info][os] Use of pthread_condattr_setclock is not supported
[0.016s][info][os] Relative timed-wait using pthread_cond_timedwait is associated with the default clock
[0.017s][info][os] SafePoint Polling address, bad (protected) page:0x0000000108901000, good (unprotected) page:0x0000000108902000
[0.022s][info][biasedlocking] Aligned thread 0x00007f983e008200 to 0x00007f983e008800
[0.023s][info][os,thread    ] Thread attached (tid: 10499, pthread id: 123145571979264).

The log is very long, not all of it is listed here.From the output log, we can see that the JVM performs many operations in the java-verson command.

We can see that the log lists the time spent on each operation, the log level, and the classification to which the operation belongs.

Through these logs, we can have a better understanding of how the JVM works.

Using Java -Xlog:helpCommand us to look at the basic format of xlog:

-Xlog Usage: -Xlog[:[selections][:[output][:[decorators][:output-options]]]]
	 where 'selections' are combinations of tags and levels of the form tag1[+tag2...][*][=level][,...]
	 NOTE: Unless wildcard (*) is specified, only log messages tagged with exactly the tags specified will be matched.

selections

selections represent exactly what information needs to be output.Is expressed as tag=level.

Tags represent events or subsystems in the JVM:

Available log tags:
 add, age, alloc, annotation, aot, arguments, attach, barrier, biasedlocking, blocks, bot, breakpoint, bytecode, cds, census, class, classhisto, cleanup, codecache, compaction, compilation, constantpool, constraints, container, coops, cpu, cset, data, datacreation, dcmd, decoder, defaultmethods, director, dump, dynamic, ergo, event, exceptions, exit, fingerprint, free, freelist, gc, handshake, hashtables, heap, humongous, ihop, iklass, init, inlining, install, interpreter, itables, jfr, jit, jni, jvmti, liveness, load, loader, logging, malloc, mark, marking, membername, memops, metadata, metaspace, methodcomparator, mirror, mmu, module, monitorinflation, monitormismatch, nestmates, nmethod, normalize, numa, objecttagging, obsolete, oldobject, oom, oopmap, oops, oopstorage, os, pagesize, parser, patch, path, perf, periodic, phases, plab, preorder, preview, promotion, protectiondomain, ptrqueue, purge, record, redefine, ref, refine, region, reloc, remset, resolve, safepoint, sampling, scavenge, setting, smr, stackmap, stacktrace, stackwalk, start, startuptime, state, stats, streaming, stringdedup, stringtable, subclass, survivor, sweep, symboltable, system, table, task, thread, time, timer, tlab, tracking, unload, unshareable, update, verification, verify, vmmutex, vmoperation, vmthread, vtables, vtablestubs, workgang
 Specifying 'all' instead of a tag combination matches all tag combinations

levels represent the level of logging:

Available log levels:
 off, trace, debug, info, warning, error

Here is an example:

java -Xlog:os,class=info -version

Output results:

[0.002s][info][os] Use of CLOCK_MONOTONIC is supported
[0.002s][info][os] Use of pthread_condattr_setclock is not supported
[0.002s][info][os] Relative timed-wait using pthread_cond_timedwait is associated with the default clock
[0.003s][info][os] SafePoint Polling address, bad (protected) page:0x0000000109543000, good (unprotected) page:0x0000000109544000
[0.006s][info][os] attempting shared library load of /Library/Java/JavaVirtualMachines/jdk-14.0.1.jdk/Contents/Home/lib/libjava.dylib
[0.007s][info][os] shared library load of /Library/Java/JavaVirtualMachines/jdk-14.0.1.jdk/Contents/Home/lib/libjava.dylib was successful
[0.007s][info][os] attempting shared library load of /Library/Java/JavaVirtualMachines/jdk-14.0.1.jdk/Contents/Home/lib/libzip.dylib
[0.010s][info][os] shared library load of /Library/Java/JavaVirtualMachines/jdk-14.0.1.jdk/Contents/Home/lib/libzip.dylib was successful

output

Output indicates where to output the log.

Optional output:

 stdout/stderr
 file=<filename>

stdout for standard output and stderr for standard error.File means output to a file.

For instance:

java -Xlog:all=debug:file=debug.log -version

decorators

decorators indicate what is output to the log.

time (t), utctime (utc), uptime (u), timemillis (tm), uptimemillis (um), timenanos (tn), uptimenanos (un), hostname (hn), pid (p), tid (ti), level (l), tags (tg)
 Decorators can also be specified as 'none' for no decoration

Look at this example:

 java -Xlog:gc*=debug:stdout:time,uptimemillis,tid -version

Output results:

 [2020-05-05T16:12:06.871-0800][32ms][9475] Heap region size: 1M
[2020-05-05T16:12:06.871-0800][32ms][9475] Minimum heap 8388608  Initial heap 134217728  Maximum heap 2147483648
[2020-05-05T16:12:06.872-0800][33ms][9475] Heap address: 0x0000000780000000, size: 2048 MB, Compressed Oops mode: Zero based, Oop shift amount: 3
[2020-05-05T16:12:06.872-0800][33ms][9475] ConcGCThreads: 1 offset 8
[2020-05-05T16:12:06.872-0800][33ms][9475] ParallelGCThreads: 4

summary

xlog is a very useful feature provided in JDK9.You can use it in your daily work.

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