JVM参数

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Trace跟踪参数

n-verbose:gc

n-XX:+printGC

n可以打印GC的简要信息

–[GC4790K->374K(15872K), 0.0001606 secs]

–[GC4790K->374K(15872K), 0.0001474 secs]

–[GC4790K->374K(15872K), 0.0001563 secs]

–[GC4790K->374K(15872K), 0.0001682 secs]

n-XX:+PrintGCDetails

–打印GC详细信息

n-XX:+PrintGCTimeStamps

–打印CG发生的时间戳

n例

–[GC[DefNew: 4416K->0K(4928K), 0.0001897 secs] 4790K->374K(15872K), 0.0002232 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

n-XX:+PrintGCDetails的输出

–Heap

– def new generation total 13824K, used 11223K [0x27e80000,0x28d80000, 0x28d80000)

– eden space 12288K, 91% used [0x27e80000, 0x28975f20, 0x28a80000)

– from space 1536K, 0% used[0x28a80000, 0x28a80000, 0x28c00000)

– to space 1536K, 0% used [0x28c00000, 0x28c00000, 0x28d80000)

– tenured generation total 5120K, used 0K [0x28d80000,0x29280000, 0x34680000)

– the space 5120K, 0% used[0x28d80000, 0x28d80000, 0x28d80200, 0x29280000)

– compacting perm gen total 12288K, used 142K [0x34680000,0x35280000, 0x38680000)

– the space 12288K, 1% used[0x34680000, 0x346a3a90, 0x346a3c00, 0x35280000)

– ro space 10240K, 44% used [0x38680000, 0x38af73f0, 0x38af7400,0x39080000)

– rw space 12288K, 52% used [0x39080000, 0x396cdd28, 0x396cde00,0x39c80000)

n-Xloggc:log/gc.log

–指定GC log的位置，以文件输出

–帮助开发人员分析问题

n-XX:+PrintHeapAtGC

–每次一次GC后，都打印堆信息

{Heapbefore GC invocations=0 (full 0):

def new generation total 3072K, used 2752K [0x33c80000,0x33fd0000, 0x33fd0000)

eden space 2752K, 100% used[0x33c80000, 0x33f30000, 0x33f30000)

from space 320K, 0% used [0x33f30000, 0x33f30000, 0x33f80000)

to space 320K, 0% used [0x33f80000,0x33f80000, 0x33fd0000)

tenured generation total 6848K, used 0K [0x33fd0000,0x34680000, 0x34680000)

the space 6848K, 0% used [0x33fd0000, 0x33fd0000, 0x33fd0200,0x34680000)

compacting perm gen total 12288K, used 143K [0x34680000,0x35280000, 0x38680000)

the space 12288K, 1% used [0x34680000, 0x346a3c58, 0x346a3e00,0x35280000)

ro space 10240K, 44% used [0x38680000, 0x38af73f0, 0x38af7400,0x39080000)

rw space 12288K, 52% used [0x39080000, 0x396cdd28, 0x396cde00,0x39c80000)

[GC[DefNew:2752K->320K(3072K), 0.0014296 secs] 2752K->377K(9920K), 0.0014604 secs][Times: user=0.00 sys=0.00, real=0.00 secs]

Heapafter GC invocations=1 (full 0):

def new generation total 3072K, used 320K [0x33c80000,0x33fd0000, 0x33fd0000)

eden space 2752K, 0% used [0x33c80000, 0x33c80000, 0x33f30000)

from space 320K, 100% used [0x33f80000,0x33fd0000, 0x33fd0000)

to space 320K, 0% used [0x33f30000,0x33f30000, 0x33f80000)

tenured generation total 6848K, used 57K [0x33fd0000,0x34680000, 0x34680000)

the space 6848K, 0% used [0x33fd0000, 0x33fde458, 0x33fde600,0x34680000)

compacting perm gen total 12288K, used 143K [0x34680000,0x35280000, 0x38680000)

the space 12288K, 1% used [0x34680000, 0x346a3c58, 0x346a3e00,0x35280000)

ro space 10240K, 44% used [0x38680000, 0x38af73f0, 0x38af7400,0x39080000)

rw space 12288K, 52% used [0x39080000, 0x396cdd28, 0x396cde00,0x39c80000)

}

n-XX:+TraceClassLoading

–监控类的加载

•[Loaded java.lang.Object from shared objects file]

•[Loaded java.io.Serializable from shared objects file]

•[Loaded java.lang.Comparable from shared objects file]

•[Loaded java.lang.CharSequence from shared objects file]

•[Loaded java.lang.String from shared objects file]

•[Loaded java.lang.reflect.GenericDeclaration from shared objects file]

•[Loaded java.lang.reflect.Type from shared objects file]

堆的分配参数

n-Xmx –Xms

–指定最大堆和最小堆

–-Xmx20m -Xms5m 运行代码：

•System.out.print("Xmx=");

•System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");

•

•System.out.print("free mem=");

•System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");

•

•System.out.print("total mem=");

•System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");

n-Xmx20m -Xms5m 运行代码

•byte[] b=new byte[1*1024*1024];

•System.out.println("分配了1M空间给数组");

•

•System.out.print("Xmx=");

•System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");

•

•System.out.print("free mem=");

•System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");

•

•System.out.print("total mem=");

•System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");

分配了1M空间给数组

Xmx=19.375M

free mem=3.4791183471679688M

total mem=4.875M

分配了1M空间给数组Xmx=19.375Mfree mem=3.4791183471679688Mtotal mem=4.875M

n-Xmx20m -Xms5m 运行代码

•b=new byte[4*1024*1024];

•System.out.println("分配了4M空间给数组");

•

•System.out.print("Xmx=");

•System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");

•

•System.out.print("free mem=");

•System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");

•

•System.out.print("total mem=");

•System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");

分配了4M空间给数组Xmx=19.375Mfree mem=3.5899810791015625Mtotal mem=9.00390625M

n-Xmx20m -Xms5m 运行代码

•System.gc();

•

•System.out.println("回收内存");

•System.out.print("Xmx=");

•System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");

•

•System.out.print("free mem=");

•System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");

•

•System.out.print("total mem=");

•System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");

回收内存Xmx=19.375Mfree mem=6.354591369628906Mtotal mem=10.75390625M

n-Xmn

–设置新生代大小

n-XX:NewRatio

–新生代（eden+2*s）和老年代（不包含永久区）的比值

–4 表示新生代:老年代=1:4，即年轻代占堆的1/5

n-XX:SurvivorRatio

–设置两个Survivor区和eden的比

–8表示两个Survivor :eden=2:8，即一个Survivor占年轻代的1/10

n根据实际事情调整新生代和幸存代的大小

n官方推荐新生代占堆的3/8

n幸存代占新生代的1/10

n在OOM时，记得Dump出堆，确保可以排查现场问题

永久区分配参数

n-XX:PermSize -XX:MaxPermSize

–设置永久区的初始空间和最大空间

–他们表示，一个系统可以容纳多少个类型

n-Xss

–通常只有几百K

–决定了函数调用的深度

–每个线程都有独立的栈空间

–局部变量、参数分配在栈上

public class TestStackDeep {

privatestatic int count=0;

publicstatic void recursion(long a,longb,long c){

longe=1,f=2,g=3,h=4,i=5,k=6,q=7,x=8,y=9,z=10;

count++;

recursion(a,b,c);

}

publicstatic void main(String args[]){

try{

recursion(0L,0L,0L);

}catch(Throwablee){

System.out.println("deep of calling = "+count);

e.printStackTrace();

}

递归调用-Xss128Kdeep of calling = 701java.lang.StackOverflowError-Xss256Kdeep of calling = 1817java.lang.StackOverflowError

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