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java | StringTable 垃圾回收

发表于 分类于 ， ， ， ， Valine：

StringTable 是会存在垃圾回收的。

使用 VM 参数 -Xmx20m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc 打印垃圾回收。

初始代码

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package com.redisc;

import lombok.extern.slf4j.Slf4j;

/**
 * 演示 StringTable 垃圾回收
 * -Xmx30m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc
 */

@Slf4j(topic = "c.Run")
public class Run {

    public static void main(String[] args) {
        int i = 0;
        try {
        } catch (Throwable e) {
            e.printStackTrace();
        } finally {
            System.out.println(i);
        }
    }

}

输出

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0
Heap
 PSYoungGen      total 9216K, used 6087K [0x00000007bf600000, 0x00000007c0000000, 0x00000007c0000000)
  eden space 8192K, 74% used [0x00000007bf600000,0x00000007bfbf1c88,0x00000007bfe00000)
  from space 1024K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007c0000000)
  to   space 1024K, 0% used [0x00000007bfe00000,0x00000007bfe00000,0x00000007bff00000)
 ParOldGen       total 20480K, used 0K [0x00000007be200000, 0x00000007bf600000, 0x00000007bf600000)
  object space 20480K, 0% used [0x00000007be200000,0x00000007be200000,0x00000007bf600000)
 Metaspace       used 4889K, capacity 5064K, committed 5248K, reserved 1056768K
  class space    used 532K, capacity 564K, committed 640K, reserved 1048576K
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, avg   8.000
Number of entries       :     18966 =    455184 bytes, avg  24.000
Number of literals      :     18966 =    717032 bytes, avg  37.806
Total footprint         :           =   1332304 bytes
Average bucket size     :     0.948
Variance of bucket size :     0.944
Std. dev. of bucket size:     0.971
Maximum bucket size     :         7
StringTable statistics:
Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      2057 =     49368 bytes, avg  24.000
Number of literals      :      2057 =    134528 bytes, avg  65.400
Total footprint         :           =    664000 bytes
Average bucket size     :     0.034
Variance of bucket size :     0.034
Std. dev. of bucket size:     0.185
Maximum bucket size     :         2

里面有一个 

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Number of entries       :      2057 =     49080 bytes, avg  24.000
Number of literals      :      2057 =    133688 bytes, avg  65.373

就是 StringTable 里面的长度

增加 StringTable

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package com.redisc;

import lombok.extern.slf4j.Slf4j;

/**
 * 演示 StringTable 垃圾回收
 * -Xmx30m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc
 */

@Slf4j(topic = "c.Run")
public class Run {

    public static void main(String[] args) {
        int i = 0;
        try {
            for (int j = 0; j < 100; j++) {
                String.valueOf(j).intern();
                i++;
            }
        } catch (Throwable e) {
            e.printStackTrace();
        } finally {
            System.out.println(i);
        }
    }

}

输出

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100
Heap
 PSYoungGen      total 9216K, used 6009K [0x00000007bf600000, 0x00000007c0000000, 0x00000007c0000000)
  eden space 8192K, 73% used [0x00000007bf600000,0x00000007bfbde578,0x00000007bfe00000)
  from space 1024K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007c0000000)
  to   space 1024K, 0% used [0x00000007bfe00000,0x00000007bfe00000,0x00000007bff00000)
 ParOldGen       total 20480K, used 0K [0x00000007be200000, 0x00000007bf600000, 0x00000007bf600000)
  object space 20480K, 0% used [0x00000007be200000,0x00000007be200000,0x00000007bf600000)
 Metaspace       used 4894K, capacity 5068K, committed 5248K, reserved 1056768K
  class space    used 532K, capacity 564K, committed 640K, reserved 1048576K
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, avg   8.000
Number of entries       :     18967 =    455208 bytes, avg  24.000
Number of literals      :     18967 =    717048 bytes, avg  37.805
Total footprint         :           =   1332344 bytes
Average bucket size     :     0.948
Variance of bucket size :     0.944
Std. dev. of bucket size:     0.971
Maximum bucket size     :         7
StringTable statistics:
Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      2145 =     51480 bytes, avg  24.000
Number of literals      :      2145 =    138760 bytes, avg  64.690
Total footprint         :           =    670344 bytes
Average bucket size     :     0.036
Variance of bucket size :     0.036
Std. dev. of bucket size:     0.189
Maximum bucket size     :         2

可以看出之前的地方变成了

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Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      2145 =     51480 bytes, avg  24.000
Number of literals      :      2145 =    138760 bytes, avg  64.690
Total footprint         :           =    670344 bytes

比之前多了接近 100 个。

大规模 StringTable

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package com.redisc;

import lombok.extern.slf4j.Slf4j;

/**
 * 演示 StringTable 垃圾回收
 * -Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc
 */

@Slf4j(topic = "c.Run")
public class Run {

    public static void main(String[] args) {
        int i = 0;
        try {
            for (int j = 0; j < 200000; j++) {
                String.valueOf(j).intern();
                i++;
            }
        } catch (Throwable e) {
            e.printStackTrace();
        } finally {
            System.out.println(i);
        }
    }

}

输出

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[GC (Allocation Failure) [PSYoungGen: 8192K->992K(9216K)] 8192K->1345K(29696K), 0.0017253 secs] [Times: user=0.01 sys=0.01, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 9184K->1024K(9216K)] 9537K->1385K(29696K), 0.0030589 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 
200000
Heap
 PSYoungGen      total 9216K, used 1427K [0x00000007bf600000, 0x00000007c0000000, 0x00000007c0000000)
  eden space 8192K, 4% used [0x00000007bf600000,0x00000007bf664cb0,0x00000007bfe00000)
  from space 1024K, 100% used [0x00000007bff00000,0x00000007c0000000,0x00000007c0000000)
  to   space 1024K, 0% used [0x00000007bfe00000,0x00000007bfe00000,0x00000007bff00000)
 ParOldGen       total 20480K, used 361K [0x00000007be200000, 0x00000007bf600000, 0x00000007bf600000)
  object space 20480K, 1% used [0x00000007be200000,0x00000007be25a4d0,0x00000007bf600000)
 Metaspace       used 4927K, capacity 5068K, committed 5248K, reserved 1056768K
  class space    used 533K, capacity 564K, committed 640K, reserved 1048576K
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, avg   8.000
Number of entries       :     19047 =    457128 bytes, avg  24.000
Number of literals      :     19047 =    719560 bytes, avg  37.778
Total footprint         :           =   1336776 bytes
Average bucket size     :     0.952
Variance of bucket size :     0.948
Std. dev. of bucket size:     0.974
Maximum bucket size     :         7
StringTable statistics:
Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      7437 =    178488 bytes, avg  24.000
Number of literals      :      7437 =    436696 bytes, avg  58.719
Total footprint         :           =   1095288 bytes
Average bucket size     :     0.124
Variance of bucket size :     0.127
Std. dev. of bucket size:     0.356
Maximum bucket size     :         3

StringTable 的数量是

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Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      7437 =    178488 bytes, avg  24.000
Number of literals      :      7437 =    436696 bytes, avg  58.719
Total footprint         :           =   1095288 bytes

远小于 200000 的数量，这是因为开头就进行了垃圾回收

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[GC (Allocation Failure) [PSYoungGen: 8192K->992K(9216K)] 8192K->1345K(29696K), 0.0017253 secs] [Times: user=0.01 sys=0.01, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 9184K->1024K(9216K)] 9537K->1385K(29696K), 0.0030589 secs] [Times: user=0.01 sys=0.00, real=0.01 secs]
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