JVM 和Java GC 笔记

学习材料

• 讲义地址： http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html
  
• Youtube 视频地址： Video The JVM and Java Garbage Collection

Java & JVM概述

1. Garbage Collection is automatic.
2. Java source code is compiled into byte code.
3. Byte code is stored in .class files
4. .class files are loaded into a Java Virtual Machine(JVM) and executed.
5. A seperated JVM is created for each Java application. (备注：！每个Java程序都对应着一个单独的JVM)

GC 的职责

1. 为新对象分配memory
2. 确保被引用的对象留在memory Ensuring that any referenced objects(live objects) remain in memory
3. 回收死掉的对象占用的memory Recovering memory used by objects that no longer reachable(dead objects)

GC的stages

step 1. marking(标记将被删除的对象)
step 2. Normal Deletion/sweeping(删除标记的对象)
step 3. Deletion with Compacting (整理内存，把碎片归拢)

Generational Collection

出发点：Java中绝大多数对象的生存周期很短。 因此按generation 来运行GC, 可以将memory 分为三部分：

Young Generation(for young objs)

• Eden
• A “from” survivor space (S0)
• A “to” survivor space (S1)

Tenured (old) Generation

• for old objs
• 超过了Minor GC age theshold的obj, 被挪到这里

Premanent Generation

• for meta data, classes, and so on
• Contains metadata required by the JVM
• Class objs and methods

Minor GC

1. 发生在Young Generation，频繁发生
2. fast，efficient。因为young gen space 通常很小，而且包含很多短命的obj
3. 熬过几次minor gc的obj，将被 promote 到old generation space

Major GC

1. 发生在old Generation的GC
2. old generation space 比yong gen 大，被占用的space 缓慢增长
3. infrequently, 而且花费的时间远多于 minor gc

Aging Obj in Yong Gen

1. 新Obj 被分配到eden space
2. 当eden space 满，触发minor GC: “ Stop the world “ event (all the application threads stop)
3. eden space满，则运行GC，把eden space 中活下来的obj + survivor space 中活下来的obj 移到另一个survivor space(反复来回倒), 并把这些obj 的age + 1
4. 如果obj的age 超过threshold(一般为15)， 将它挪到Old Gen

感受

除了官方的文档、视频，Oracle JDK还提供了demos 和samples, 自己可以实际操作，加深感受。