GC介绍 java堆内存空间 查看jvm默认垃圾收集器 java -XX:+PrintCommandLineFlags -version java -XX:+PrintGCDetails -version java -XX:+PrintFlagsFinal -version 几种经典的垃圾回收器 Serial ParNew Parallel Scavenge Serial Old Parallel Old CMS(Concurrent Mark Sweep) G1(Garbage First) ZGC