JDK9的新特性:JVM的xlog

时间 2020-05-24

标签 jdk9 jdk 特性 jvm xlog 栏目 Java 繁體版

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简介

在java程序中，咱们经过日志来定位和发现项目中可能出现的问题。在现代java项目中，咱们使用log4j或者slf4j，Logback等日志记录框架来处理日志问题。java

JVM是java程序运行的基础，JVM中各类事件好比：GC,class loading,JPMS,heap,thread等等其实均可以有日志来记录。经过这些日志，咱们能够监控JVM中的事件，并能够依次来对java应用程序进行调优。程序员

在JDK9中引入的Xlog日志服务就是为这个目的而建立的。segmentfault

经过xlog，JDK将JVM中的各类事件统一块儿来，以统一的形式对外输出。经过tag参数来区分子系统，经过log level来区分事件的紧急性，经过logging output来配置输出的地址。框架

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xlog的使用

先看一个最简单的xlog的使用例子：less

java -Xlog -version

输出结果：dom

[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).

日志很是很是长，这里就不所有列出来了。从输出的日志咱们能够看到java -verson命令中JVM执行了诸多的操做。ssh

咱们能够看到日志中对每个操做都列出了操做花费的时间，日志级别和操做所属的分类。jvm

经过这些日志，咱们对于JVM的运行能够有更加深刻的理解。oop

使用java -Xlog:help命令咱们看一下xlog的基本格式：区块链

-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表示的是到底须要输出哪些信息。是以tag=level来表示的。

tag表示的是JVM中的事件或者子系统：

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表示的是日志的级别：

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

下面举个例子：

java -Xlog:os,class=info -version

输出结果：

[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表示将日志输出到什么地方。

output的可选项：

stdout/stderr
 file=<filename>

stdout表示标准输出，stderr表示标准错误。file表示输出到文件里面。

举个例子：

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

decorators

decorators表示输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

看下这个例子：

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

输出结果：

[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

总结

xlog是JDK9中提供的很是有用的一个功能。你们能够在平常的工做中使用。

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