设置参数-Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc
/**
* 演示 StringTable 垃圾回收
* -Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc
*/
public class Demo {
public static void main(String[] args) throws InterruptedException {
int i = 0;
try {
} catch (Throwable e) {
e.printStackTrace();
} finally {
System.out.println(i);
}
}
}
打印
0
Heap
PSYoungGen total 2560K, used 1955K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 95% used [0x00000000ffd00000,0x00000000ffee8c18,0x00000000fff00000)
from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 7168K, used 0K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
object space 7168K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffd00000)
Metaspace used 3293K, capacity 4564K, committed 4864K, reserved 1056768K
class space used 361K, capacity 388K, committed 512K, reserved 1048576K
SymbolTable statistics:
Number of buckets : 20011 = 160088 bytes, avg 8.000
Number of entries : 13355 = 320520 bytes, avg 24.000
Number of literals : 13355 = 594648 bytes, avg 44.526
Total footprint : = 1075256 bytes
Average bucket size : 0.667
Variance of bucket size : 0.668
Std. dev. of bucket size: 0.817
Maximum bucket size : 6
StringTable statistics:
Number of buckets : 60013 = 480104 bytes, avg 8.000
Number of entries : 1745 = 41880 bytes, avg 24.000
Number of literals : 1745 = 177296 bytes, avg 101.602
Total footprint : = 699280 bytes
Average bucket size : 0.029
Variance of bucket size : 0.029
Std. dev. of bucket size: 0.171
Maximum bucket size : 2
重点观察其中的StringTable statistics
1.创建100个字符串(不会触发垃圾回收)
public class Demo {
public static void main(String[] args) throws InterruptedException {
int i = 0;
try {
for (int j = 0; j < 100; j++) { // j=100, j=10000,100000
String.valueOf(j).intern();
i++;
}
} catch (Throwable e) {
e.printStackTrace();
} finally {
System.out.println(i);
}`
}
}
打印
StringTable statistics:
Number of buckets : 60013 = 480104 bytes, avg 8.000
Number of entries : 1845 = 44280 bytes, avg 24.000
Number of literals : 1845 = 182096 bytes, avg 98.697
Total footprint : = 706480 bytes
Average bucket size : 0.031
Variance of bucket size : 0.031
Std. dev. of bucket size: 0.175
Maximum bucket size : 2
可以发现Number of entries与Number of literals新增了100个字符串对象
2.创建10000个字符串(触发垃圾回收)
[GC (Allocation Failure) [PSYoungGen: 2048K->488K(2560K)] 2048K->786K(9728K), 0.0359746 secs] [Times: user=0.00 sys=0.00, real=0.04 secs]
10000
Heap
PSYoungGen total 2560K, used 851K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 17% used [0x00000000ffd00000,0x00000000ffd5ac98,0x00000000fff00000)
from space 512K, 95% used [0x00000000fff00000,0x00000000fff7a020,0x00000000fff80000)
to space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
ParOldGen total 7168K, used 298K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
object space 7168K, 4% used [0x00000000ff600000,0x00000000ff64a8b0,0x00000000ffd00000)
Metaspace used 3296K, capacity 4564K, committed 4864K, reserved 1056768K
class space used 361K, capacity 388K, committed 512K, reserved 1048576K
SymbolTable statistics:
Number of buckets : 20011 = 160088 bytes, avg 8.000
Number of entries : 13356 = 320544 bytes, avg 24.000
Number of literals : 13356 = 594664 bytes, avg 44.524
Total footprint : = 1075296 bytes
Average bucket size : 0.667
Variance of bucket size : 0.668
Std. dev. of bucket size: 0.817
Maximum bucket size : 6
StringTable statistics:
Number of buckets : 60013 = 480104 bytes, avg 8.000
Number of entries : 8582 = 205968 bytes, avg 24.000
Number of literals : 8582 = 505552 bytes, avg 58.908
Total footprint : = 1191624 bytes
Average bucket size : 0.143
Variance of bucket size : 0.154
Std. dev. of bucket size: 0.393
Maximum bucket size : 3
因为创建的字符串对象没有被引用，所以无用的字符串被垃圾回收