【JVM】GC（四）：GC日志格式解析

向右看齐 2022-10-30 13:25 264阅读 0赞

后面会结合实例来说明，先准备一个jar包，这里我拿的是一个启动 eureka 的 SpringBoot 项目。

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那下面我们就分别配置不同的垃圾收集器，然后分析它们的日志内容。

> PS：打印GC日志的JVM参数是：`-XX:+PrintGCDetails` `-XX:+PrintGCTimeStamps` `-XX:+PrintGCDateStamps` `-Xloggc:./gc.log -jar`

### 1.Parallel Scavenge收集器(默认) ###

因为 Parallel Scavenge 是默认收集器，所以只用在启动时指定打印GC日志即可

java -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -Xloggc:./gc.log -jar eureka-server.jar

[GC (Allocation Failure) 
    [PSYoungGen: 33280K[Young区回收前]->3123K[Young区回收后] (38400K[Young区总大小])]
    33280K[整个堆回收前]->3139K[整个堆回收后](125952K[整个堆总大小]), 0.0158731 secs] 
    [Times: user=0.02 sys=0.00, real=0.02 secs]

### 2.CMS收集器 ###

CMS收集器需要在启动时手动设置

java -XX:+UseConcMarkSweepGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -Xloggc:./gc-cms.log -jar eureka-server.jar

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 4]

在桌面打gc-cms.log如下：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 5]  
对于青年代 ParNew 每行的日志内容与 Parallel Scavenge 类似，因为两者都是并行收集器，这里就不说了。但是，在老年代就还可以看到CMS的垃圾回收的过程：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 6]

### 3.G1收集器 ###

G1是一个同时可以处理青年代与老年代的收集器，若要使用G1收集器，也需要在启动项目时手动配置：-XX:+UseG1GC

java -XX:+UseG1GC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -Xloggc:./gc-g1.log -jar eureka-server.jar

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 9]  
由于示例代码中并未出现FullGC，所以这里就不演示了。更多G1先关格式解析可参考：[Understanding G1 GC Logs][]…

> PS：GC 日志其实一般是用来 GC 调优的，关于 GC 调优可以参考我的[这篇文章][Link 1]…

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[Understanding G1 GC Logs]: https://blogs.oracle.com/poonam/understanding-g1-gc-logs
[Link 1]: https://blog.csdn.net/weixin_43935927/article/details/109232363