JDK9的新特性:JVM的xlog

目錄

• ​​簡介​​
• ​​xlog的使用​​
• ​​selections​​
• ​​output​​
• ​​decorators​​
• ​​總結​​

簡介

在java程式中，我們通過日志來定位和發現項目中可能出現的問題。在現代java項目中，我們使用log4j或者slf4j，Logback等日志記錄架構來處理日志問題。

JVM是java程式運作的基礎，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，并可以依次來對java應用程式進行調優。

在JDK9中引入的Xlog日志服務就是為這個目的而建立的。

通過xlog，JDK将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來配置輸出的位址。

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了。從輸出的日志我們可以看到java -verson指令中JVM執行了諸多的操作。

我們可以看到日志中對每一個操作都列出了操作花費的時間，日志級别和操作所屬的分類。

通過這些日志，我們對于JVM的運作可以有更加深入的了解。

使用java -Xlog:help指令我們看一下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資訊。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表示的是日志的級别：

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表示将日志輸出到什麼地方。

output的可選項：

stdout/stderr
 file=<filename>      

stdout表示标準輸出，stderr表示标準錯誤。file表示輸出到檔案裡面。

舉個例子：

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

decorators

decorators表示輸出哪些内容到日志中。

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      

總結

xlog是JDK9中提供的非常有用的一個功能。大家可以在日常的工作中使用。

本文作者：flydean程式那些事