簡(jiǎn)介

JVM是java程序運(yùn)行的基礎(chǔ)，JVM中各種事件比如：GC,class loading,JPMS,heap,thread等等其實(shí)都可以有日志來(lái)記錄。通過(guò)這些日志，我們可以監(jiān)控JVM中的事件，并可以依次來(lái)對(duì)java應(yīng)用程序進(jìn)行調(diào)優(yōu)。

在JDK9中引入的Xlog日志服務(wù)就是為這個(gè)目的而創(chuàng)建的。

通過(guò)xlog，JDK將JVM中的各種事件統(tǒng)一起來(lái)，以統(tǒng)一的形式對(duì)外輸出。通過(guò)tag參數(shù)來(lái)區(qū)分子系統(tǒng)，通過(guò)log level來(lái)區(qū)分事件的緊急性，通過(guò)logging output來(lái)配置輸出的地址。

xlog的使用

先看一個(gè)最簡(jiǎn)單的xlog的使用例子：

java -Xlog -version

輸出結(jié)果：

[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).

日志非常非常長(zhǎng)，這里就不全部列出來(lái)了。從輸出的日志我們可以看到j(luò)ava -verson命令中JVM執(zhí)行了諸多的操作。

我們可以看到日志中對(duì)每一個(gè)操作都列出了操作花費(fèi)的時(shí)間，日志級(jí)別和操作所屬的分類。

通過(guò)這些日志，我們對(duì)于JVM的運(yùn)行可以有更加深入的理解。

使用java -Xlog:help命令我們看一下xlog的基本格式：

-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表示的是到底需要輸出哪些信息。是以tag=level來(lái)表示的。

tag表示的是JVM中的事件或者子系統(tǒng)：

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表示的是日志的級(jí)別：

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

下面舉個(gè)例子：

java -Xlog:os,class=info -version

輸出結(jié)果：

[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表示將日志輸出到什么地方。

output的可選項(xiàng)：

stdout/stderr file=<filename>

stdout表示標(biāo)準(zhǔn)輸出，stderr表示標(biāo)準(zhǔn)錯(cuò)誤。file表示輸出到文件里面。

舉個(gè)例子：

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

decorators

decorators表示輸出哪些內(nèi)容到日志中。

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

看下這個(gè)例子：

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

輸出結(jié)果：

[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

以上就是淺談JDK9的特性之JVM的xlog的詳細(xì)內(nèi)容，更多關(guān)于JDK9的特性之JVM的xlog的資料請(qǐng)關(guān)注腳本之家其它相關(guān)文章！

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