内存 ：可以选择查看“堆内存使用情况”、“非堆内存使用情况”、“内存池”PS Eden Space””等内存占用的实时情况；界面右下角还有图形化的堆一级、二级、三级缓存（从左到右）占用情况，当然，如果三级缓存被全部占用也就是很可能内存溢出啦！这时可以去查看服务器的tomcat日志，应该会有“outofmemory”的异常日志信息。界面右上角处还提供了一个“执行GC”的手动垃圾收集功能，这个也很实用~而且界面下方还有详细的GC信息记录。，整个界面提供了关于垃圾收集必须的各项基础指标查询。
Description
Java VisualVM is an intuitive graphical user interface that provides detailed information about Java technology-based applications (Java applications) while they are running on a specified Java Virtual Machine (JVM). The name Java VisualVM comes from the fact that Java VisualVM provides information about the JVM software visually.
Java VisualVM combines several monitoring, troubleshooting, and profiling utilities into a single tool. For example, most of the functionality offered by the standalone tools jmap, jinfo, jstat, and jstack were integrated into Java VisualVM. Other functionality, such as some that offered by the jconsole command, can be added as optional plug-ins.
Java VisualVM is useful to Java application developers to troubleshoot applications and to monitor and improve the applications' performance. Java VisualVM enables developers to generate and analyze heap dumps, track down memory leaks, perform and monitor garbage collection, and perform lightweight memory and CPU profiling. You can expand the Java VisualVM functionality with plug-ins. For example, most of the functionality of the jconsole command is available through the MBeans Tab and JConsole Plug-in Wrapper plug-ins. You can choose from a catalog of standard Java VisualVM plug-ins by selecting Tools and then Plugins in the Java VisualVM menus.
Start Java VisualVM with the following command:
% jvisualvm <options>
-XX:+UseG1GC # 使用了G1垃圾收集器# 什么时候发生的GC，相对的时间刻，GC发生的区域young，总共花费的时间，0.00478s，# It is a stop-the-world activity and all# the application threads are stopped at a safepoint during this time.2019-12-18T16:06:46.508+0800: 0.458: [GC pause (G1 Evacuation Pause)(young), 0.0047804 secs]# 多少个垃圾回收线程，并行的时间[Parallel Time: 3.0 ms, GC Workers: 4]# GC线程开始相对于上面的0.458的时间刻[GC Worker Start (ms): Min: 458.5, Avg: 458.5, Max: 458.5, Diff: 0.0]# This gives us the time spent by each worker thread scanning the roots# (globals, registers, thread stacks and VM data structures).[Ext Root Scanning (ms): Min: 0.2, Avg: 0.4, Max: 0.7, Diff: 0.5, Sum: 1.7]# Update RS gives us the time each thread spent in updating the RememberedSets.[Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]...# 主要是Eden区变大了，进行了调整[Eden: 14.0M(14.0M)->0.0B(16.0M) Survivors: 0.0B->2048.0K Heap:14.0M(256.0M)->3752.5K(256.0M)]
