java堆内存怎么手工回收,Java堆内存分配和回收实践

悠悠 2023-01-13 09:23 79阅读 0赞

Java运行期的内存结构包括堆(Heap)、栈(VM Stack)、方法区(Method Aread)、本地方法栈(Native Method Stack)、程序计数器(Pragram Counter Register)这几部分，其中和我们日常编程最密切的就是堆(Heap)了。

堆(Heap)又分为三部分，Young(New)、Tenured(Old)、Perm，对应的中文名叫法分别为新生代、老年代、永久代，而新生代又可以为分Eden、From Survivor、To Survivor，更详细的内存结构说明和GC回收算法请参考这篇文章《JDK5.0中JVM堆模型、GC垃圾收集详细解析》。

为了更深入地理解内存回收机制，下面的代码是我参照《深入理解JAVA虚拟机-JVM高级特性与最佳实践》提供的例子而写，分别触发Minor GC和Full GC。

package org.bocai.jvm.gc;

/\*\*

\* VM Params:-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:SurvivorRatio=8 -XX:+PrintGCDetails -XX:MaxTenuringThreshold=1 -XX:+PrintTenuringDistribution

\* @author yikebocai@gmail.com

\* @since 2013-3-20

\*/

public class GCDemo \{

private static final int \_1MB=1024\*1024;

public static void main(String\[\] args) \{

testMinorGC();

/\*\*

\* Minor GC Demo

\*/

private static void testMinorGC() \{

byte\[\] alloc1=new byte\[2\*\_1MB\];

byte\[\] alloc2=new byte\[2\*\_1MB\];

byte\[\] alloc3=new byte\[1\*\_1MB\];

//first Minor GC,5M Eden->Old

byte\[\] alloc4=new byte\[4\*\_1MB\];

byte\[\] alloc5=new byte\[\_1MB/4\];

//second Minor GC,alloc5:Eden->Surivior

byte\[\] alloc6=new byte\[4\*\_1MB\];

//if MaxTenuringThreshold>2 ,then move alloc5 into old,but whatever alloc5 is moved into old,why?

alloc4=null;

byte\[\] alloc7=new byte\[4\*\_1MB\];

//System.out.println(alloc5.length);

/\*\*

\* Full GC Demo

\*/

private static void testFullGC() \{

byte\[\] alloc1=new byte\[2\*\_1MB\];

byte\[\] alloc2=new byte\[2\*\_1MB\];

byte\[\] alloc3=new byte\[1\*\_1MB\];

//first Minor GC

byte\[\] alloc4=new byte\[4\*\_1MB\];

byte\[\] alloc5=new byte\[3\*\_1MB\];

//second Minor GC

byte\[\] alloc6=new byte\[4\*\_1MB\];

alloc1=null;

alloc2=null;

//first Full GC

byte\[\] alloc7=new byte\[2\*\_1MB\];

上面的例子很简单，我的困惑来自于Surivivor的对象处理机制，如果MaxTenuringThreshold大于1,在分配alloc7对象时,alloc5对象只经历了一次Minor GC,还没有达到MaxTenuringThreshold设定的值,为何放在Survivor区的alloc5对象就被移到了Old区,并且不管这个参数的值设为多少,只要经历alloc7对象的分配,都会被移到Old区,如下所示:

\[GC \[DefNew

Desired survivor size 524288 bytes, new threshold 1 (max 1)

\- age 1: 150912 bytes, 150912 total

: 5463K->147K(9216K), 0.0705580 secs\] 5463K->5267K(19456K), 0.0706022 secs\] \[Times: user=0.00 sys=0.08, real=0.07 secs\]

\[GC \[DefNew

Desired survivor size 524288 bytes, new threshold 1 (max 1)

\- age 1: 262160 bytes, 262160 total

: 4586K->256K(9216K), 0.0106228 secs\] 9706K->9619K(19456K), 0.0106669 secs\] \[Times: user=0.02 sys=0.00, real=0.01 secs\]

\[GC \[DefNew: 4352K->4352K(9216K), 0.0000259 secs\]\[Tenured: 9363K->9619K(10240K), 0.4481241 secs\] 13715K->9619K(19456K), \[Perm : 369K->369K(12288K)\], 0.4482521 secs\] \[Times: user=0.00 sys=0.03, real=0.45 secs\]

Heap

def new generation total 9216K, used 4259K \[0x322a0000, 0x32ca0000, 0x32ca0000)

eden space 8192K, 52% used \[0x322a0000, 0x326c8fe0, 0x32aa0000)

from space 1024K, 0% used \[0x32aa0000, 0x32aa0000, 0x32ba0000)

to space 1024K, 0% used \[0x32ba0000, 0x32ba0000, 0x32ca0000)

tenured generation total 10240K, used 9619K \[0x32ca0000, 0x336a0000, 0x336a0000)

the space 10240K, 93% used \[0x32ca0000, 0x33604dd0, 0x33604e00, 0x336a0000)

compacting perm gen total 12288K, used 369K \[0x336a0000, 0x342a0000, 0x376a0000)

the space 12288K, 3% used \[0x336a0000, 0x336fc718, 0x336fc800, 0x342a0000)

ro space 10240K, 51% used \[0x376a0000, 0x37bccf58, 0x37bcd000, 0x380a0000)

rw space 12288K, 54% used \[0x380a0000, 0x38738f50, 0x38739000, 0x38ca0000)

我们注意到from space 1024K,0%,Survivor区的对象被移走了,把alloc7对象分配那行注释掉,再看一下:

\[GC \[DefNew

Desired survivor size 524288 bytes, new threshold 1 (max 1)

\- age 1: 150912 bytes, 150912 total

: 5463K->147K(9216K), 0.0067181 secs\] 5463K->5267K(19456K), 0.0067671 secs\] \[Times: user=0.00 sys=0.00, real=0.01 secs\]

\[GC \[DefNew

Desired survivor size 524288 bytes, new threshold 1 (max 1)

\- age 1: 262160 bytes, 262160 total

: 4586K->256K(9216K), 0.0081092 secs\] 9706K->9619K(19456K), 0.0081469 secs\] \[Times: user=0.00 sys=0.02, real=0.01 secs\]

Heap

def new generation total 9216K, used 4515K \[0x322a0000, 0x32ca0000, 0x32ca0000)

eden space 8192K, 52% used \[0x322a0000, 0x326c8fe0, 0x32aa0000)

from space 1024K, 25% used \[0x32aa0000, 0x32ae0010, 0x32ba0000)

to space 1024K, 0% used \[0x32ba0000, 0x32ba0000, 0x32ca0000)

tenured generation total 10240K, used 9363K \[0x32ca0000, 0x336a0000, 0x336a0000)

the space 10240K, 91% used \[0x32ca0000, 0x335c4dc0, 0x335c4e00, 0x336a0000)

compacting perm gen total 12288K, used 369K \[0x336a0000, 0x342a0000, 0x376a0000)

the space 12288K, 3% used \[0x336a0000, 0x336fc700, 0x336fc800, 0x342a0000)

ro space 10240K, 51% used \[0x376a0000, 0x37bccf58, 0x37bcd000, 0x380a0000)

rw space 12288K, 54% used \[0x380a0000, 0x38738f50, 0x38739000, 0x38ca0000)

from space 1024K, 25% used里有alloc5对象,符合我们的预期,上面的情况一直没弄明白是什么原因.

java堆内存怎么手工回收,Java堆内存分配和回收实践

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