java入门教程：数据类型_设置堆的比例分配
来源 :中华考试网 2016-05-05
中设置堆的比例分配
参数-XX：SurvivorRatio 是用来设置新生代中 eden 空间和 s0 空间的比例关系。s0 和 s1 空间又分别称为 from 空间和 to 空间。它们的大小是相同的，职能也是相同的，并在 Minor GC 后互换角色。
清单 15. 所示示例演示不断插入字符时使用的 GC 输出
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import java.util.ArrayList;
import java.util.List;
public class StringDemo {
public static void main(String[] args){
List handler = new ArrayList();
for(int i=0;i<1000;i++){
HugeStr h = new HugeStr();
ImprovedHugeStr h1 = new ImprovedHugeStr();
handler.add(h.getSubString(1, 5));
handler.add(h1.getSubString(1, 5));
}
}
static class HugeStr{
private String str = new String(new char[800000]);
public String getSubString(int begin,int end){
return str.substring(begin, end);
}
}
static class ImprovedHugeStr{
private String str = new String(new char[10000000]);
public String getSubString(int begin,int end){
return new String(str.substring(begin, end));
}
}
}
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清单 16. 设置新生代堆为 10MB，并使 eden 区是 s0 的 8
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-XX:+PrintGCDetails -XX:MaxNewSize=10M -XX:SurvivorRatio=8
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清单 17. 运行输出
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[Full GC [Tenured: 233756K->233743K(251904K), 0.0524229 secs]
233756K->233743K(261120K),
[Perm : 377K->372K(12288K)], 0.0524703 secs] [Times: user=0.06 sys=0.00,
real=0.06 secs]
def new generation total 9216K, used 170K [0x27010000, 0x27a10000,
0x27a10000)
eden space 8192K, 2% used [0x27010000, 0x2703a978, 0x27810000)
from space 1024K, 0% used [0x27910000, 0x27910000, 0x27a10000)
to space 1024K, 0% used [0x27810000, 0x27810000, 0x27910000)
tenured generation total 251904K, used 233743K [0x27a10000, 0x37010000,
0x37010000)
the space 251904K, 92% used [0x27a10000, 0x35e53d00, 0x35e53e00,
0x37010000)
compacting perm gen total 12288K, used 372K [0x37010000, 0x37c10000,
0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706d310, 0x3706d400,
0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000,
0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600,
0x3c610000)
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修改参数 SurvivorRatio 为 2 运行程序，相当于设置 eden 区是 s0 的 2 倍大小，由于 s1 与 s0 相同，故有 eden=[10MB/(1+1+2)]*2=5MB。
清单 18. 运行输出
[Full GC [Tenured: 233756K->233743K(251904K), 0.0546689 secs]
233756K->233743K(259584K),
[Perm : 377K->372K(12288K)],0.0547257 secs] [Times: user=0.05 sys=0.00,
real=0.05 secs]
def new generation total 7680K, used 108K [0x27010000, 0x27a10000,
0x27a10000)
eden space 5120K, 2% used [0x27010000, 0x2702b3b0, 0x27510000)
from space 2560K, 0% used [0x27510000, 0x27510000, 0x27790000)
to space 2560K, 0% used [0x27790000, 0x27790000, 0x27a10000)
tenured generation total 251904K, used 233743K [0x27a10000, 0x37010000,
0x37010000)
the space 251904K, 92% used [0x27a10000, 0x35e53d00, 0x35e53e00,
0x37010000)
compacting perm gen total 12288K, used 372K [0x37010000, 0x37c10000,
0x3b010000)
the space 12288K, 3% used [0x37010000, 0x3706d310, 0x3706d400,
0x37c10000)
ro space 10240K, 51% used [0x3b010000, 0x3b543000, 0x3b543000,
0x3ba10000)
rw space 12288K, 55% used [0x3ba10000, 0x3c0ae4f8, 0x3c0ae600,
0x3c610000)