查看“︁JVM GC summary”︁的源代码

[[File:Hotspot-Arch.png|600px|HotSpot JVM architecture]]


= JVM Generations=

[[File:Hotspot-heap.png|600px|Hotspot Heap Structure]]

Java的堆被划分成不同的区域：

* young generation：存放新创建的对象，当这个区域占满的时候，会触发<span class="article-label">minor GC</span>，这时候存活的对象会被标记年龄，最终会移动到<span class="article-label">old generation</span>。
* old generation：存放存活的比较久的对象。当<span class="article-label">yound generation</span>存活的对象年龄到达设置的阈值后，就会被移动到这里来。当这个区域满了的时候，会触发<span class="article-label">major GC</span>。
* permanent generation：存放一些JVM运行所需的元数据，例如类的信息等。<span class="article-label">full GC</span>的时候也包括对这个区域的GC。
其中，<span class="article-label">minor GC</span>和<span class="article-label">major GC</span>都是<span class="article-label">Stop the World</span>的，即当GC触发的时候，所有的程序线程都会停止等待GC完成。通常<span class="article-label">minor GC</span>会比<span class="article-label">major GC</span>快很多，因为<span class="article-label">major GC</span>会遍历所有的存活对象。

其中，<span class="article-label">yound generation</span> 又被划分成<span class="article-label">Eden space</span>, <span class="article-label">Survivor Space1</span>, <span class="article-label">Survivor Space2</span>，其中<span class="article-label">Eden Space</span>占了绝大部分的空间。当<span class="article-label">Eden space</span>满的时候，GC 会将存活对象移动到其中一个<span class="article-label">Survivor Space</span>中，两个<span class="article-label">Survivor Space</span>是为了避免内存碎片，每次将存活的对象（Eden Space以及上一个Survivor Space）移动到另一个Survivor Space中。

[[File:Hotspot-Aging.png|600px|Mnior GC]]

通过<span class="article-label">Java VisualVM</span>和VisualGC插件可以很直观的看到GC的过程:
[[File:visualVM_GC.png|600px|Visual GC]]

<syntaxhighlight lang="bash">
java -Xmx50m \
-XX:-PrintGC \
-XX:+PrintHeapAtGC \
-XX:MaxTenuringThreshold=10 \
-XX:+UseConcMarkSweepGC \
-XX:+UseParNewGC TestGC
</syntaxhighlight>

= Garbage Collectors=

<pre>
Argument        Description
--------------- ---------------------------------------------------------
-Xms	        Sets the initial heap size for when the JVM starts.
-Xmx	        Sets the maximum heap size.
-Xmn	        Sets the size of the Young Generation.
-XX:PermSize	Sets the starting size of the Permanent Generation.
-XX:MaxPermSize	Sets the maximum size of the Permanent Generation
</pre>

;Serial GC:使用mark-compact算法进行GC，单线程的进行GC，适合单核CPU和在客户端允许的Java程序。
;Parallel GC(throughput collector):多线程进行GC
;Concurrent Mark Sweep (CMS) Collector: 在程序运行的时候并发的进行GC，以最大限度减少停止时间
;G1(Garbage-First) Garbage Collector: CMS的替代品

其中存在并行(Parallel)和并发(Concurrent)的区别，并行是指垃圾收集器多个线程同时工作，但此时用户线程依然是停止等待的；而并发是指在用户线程工作的同时，垃圾收集器同时执行。


[[File:JVM-GC-Collectors.jpg|600px|Java collectors]]
<pre>
Garbage Collector Type           Algorithm                MultiThread
----------------- -------------- ------------------------ ----------------------
Serial            stop-the-world copying                  No
ParNew            stop-the-world copying                  Yes
Parallel Scavenge stop-the-world copying                  Yes
Serial Old        stop-the-world mark-sweep-compact       No
CMS               low-pause      concurrent-mark-sweep    Yes
Parallel Old      stop-the-world mark-sweep-compact       Yes
G1                               compacting               Yes        
</pre>

<pre>
Arguments                Result
------------------------ --------------------------------------------------------
-XX:+UseSerialGC         Serial + Serial Old
-XX:+UseParNewGC         ParNew + Serial Old
-XX:+UseConcMarkSweepGC  ParNew + CMS + Serial Old^["CMS" is used most of the time to collect the tenured generation. "Serial Old" is used when a concurrent mode failure occurs.]
-XX:+UseParallelGC       Parallel Scavenge + Serial Old
-XX:+UseParallelOldGC    Parallel Scavenge + Parallel Old
–XX:+UseG1GC             G1
</pre>

= GC过程=
== CMS==
CMS 收集器的步骤：
<pre>
Phase                    Description
------------------------ -------------------------------------------------------
Initial Mark (Stop-Word) 标记老年代中的对象是否可达(reachable)，包括可以从新生代中到达的
Concurrent Marking   
Remark       (Stop-Word)
Concurrent Sweep
Resetting
</pre>
G1将Heap划分成一些相同大小的区块，但是没有限制不同代的大小。

[[File:JVM-GC-G1.png|600px|G1 Heap Allocation]]

= 其他=
当对象被GC（之前）的时候，如果没有执行<span class="article-label">finalize</span>方法，是会执行一次finalize的。如果在finalize方法里面又被强引用了，那么会阻止对象被GC。！！


References:

* [http://enos.itcollege.ee/~jpoial/allalaadimised/reading/Advanced-java.pdf Advanced Java]
* [https://codeahoy.com/2017/08/06/basics-of-java-garbage-collection/ Basics of Java Garbage Collection]
* [https://www.novatec-gmbh.de/en/blog/g1-action-better-cms/ G1 in Action: Is it better than the CMS?]
* [https://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html Java Garbage Collection Basics]
* [https://www.oracle.com/webfolder/technetwork/tutorials/obe/java/G1GettingStarted/index.html Getting Started with the G1 Garbage Collector]

[[Category:JVM]]