淺談JDK9的特性之JVM的xlog

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簡介

JVM是java程序運行的基礎，JVM中各種事件比如：GC,class loading,JPMS,heap,thread等等其實都可以有日志來記錄。通過這些日志，我們可以監控JVM中的事件，並可以依次來對java應用程序進行調優。

在JDK9中引入的Xlog日志服務就是為這個目的而創建的。

通過xlog，JDK將JVM中的各種事件統一起來，以統一的形式對外輸出。通過tag參數來區分子系統，通過log level來區分事件的緊急性，通過logging output來配置輸出的地址。

xlog的使用

先看一個最簡單的xlog的使用例子：

java -Xlog -version

輸出結果：

[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，這裡就不全部列出來瞭。從輸出的日志我們可以看到java -verson命令中JVM執行瞭諸多的操作。

我們可以看到日志中對每一個操作都列出瞭操作花費的時間，日志級別和操作所屬的分類。

通過這些日志，我們對於JVM的運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來表示的。

tag表示的是JVM中的事件或者子系統：

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表示的是日志的級別：

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

下面舉個例子：

java -Xlog:os,class=info -version

輸出結果：

[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麼地方。

output的可選項：

stdout/stderr file=<filename>

stdout表示標準輸出，stderr表示標準錯誤。file表示輸出到文件裡面。

舉個例子：

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

decorators

decorators表示輸出哪些內容到日志中。

time (t), utctime (utc), uptime (u), timemillis ™, 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

看下這個例子：

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

輸出結果：

[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的詳細內容，更多關於JDK9的特性之JVM的xlog的資料請關註WalkonNet其它相關文章！

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