JVM note 2

JVM memory

Gabage Collection

Reference Counting （Python)

Tracing Gabage Collection

finalization

MAT && JProfiler GC Root

IDEA： JProfiler

JProfiler

Mark-Sweep

Copying

Mark-Compact

Mark-Sweep vs Copying vs Mark-Compact

Generational Collecting

System.gc()

Full GC

public class SystemGCTest {
    public static void main (String[] args) {
        new SystemGCTest();
        System.gc(); // remind jvc excute gc

        System.runfinalization();
    }
    @Override
    protected void finalize() thrwows Throwable {
        super.finalize();
        System.put.println("SystemGCTest override finalize()");
    }
}

Memory leak && OOM

• OOM :没有空闲内存，并且垃圾收集器也无法提供更多内容
• OutOfMemoryError: JVM GC a soft reference object

Memory leak: A memory leak occurs when a process allocates memory from the paged or nonpaged pools, but does not free the memory

• 只有对象不会再被程序用到了，但是 GC 又不能回收它们，才叫内存泄漏

• 但是不好的实践中导致对象的生命周期变成很长导致 OOM, 也是宽泛意义上的内存泄露

Stop The World

concurrent GC && parallel GC

Concurrent

• single Processor

GC Safe Points && Safe Region

Reference Counting

• Strong Reference

Object obj = new Object() 强引用存在不回收

• Soft Reference

内存不足即回收 （Cache）不会导致 OOM

• Weak Reference

发现就回收 （Cache）

• Phantom Reference

不会对生存影响，无法获得实例，唯一目的是 gc 收到通知

GC Performance

different GC

Serial Collector

默认被应用在客户端的 Client 模式下的 JVM

ParNew Collector

Parallel Collector

G1 Collector

pros and cons

*G1 working process

G1 Remembered Set

Q1: examples to explain Memory Leak

• singleton
• close

JVM