StringTable串池垃圾回收

先看一段代码:

package com.tech.constantpool;


public class Demo_6 {
    public static void main(String[] args) {
        int c = 0;
        try {
//            for(int i=0;i<10000;i++){
//                String.valueOf(i).intern();
//                ++c;
//            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println(c);
        }


    }
}

在IDE配置JVM参数

-Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc

运行结果：

"C:Program FilesJavajdk1.8.0_251binjava.exe" -Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc "-javaagent:D:TechresourceIntelliJ IDEA 2018.3.1libidea_rt.jar=58788:D:TechresourceIntelliJ IDEA 2018.3.1bin" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.8.0_251jrelibcharsets.jar;C:Program FilesJavajdk1.8.0_251jrelibdeploy.jar;C:Program FilesJavajdk1.8.0_251jrelibextaccess-bridge-64.jar;C:Program FilesJavajdk1.8.0_251jrelibextcldrdata.jar;C:Program FilesJavajdk1.8.0_251jrelibextdnsns.jar;C:Program FilesJavajdk1.8.0_251jrelibextjaccess.jar;C:Program FilesJavajdk1.8.0_251jrelibextjfxrt.jar;C:Program FilesJavajdk1.8.0_251jrelibextlocaledata.jar;C:Program FilesJavajdk1.8.0_251jrelibextnashorn.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunec.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunmscapi.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunpkcs11.jar;C:Program FilesJavajdk1.8.0_251jrelibextzipfs.jar;C:Program FilesJavajdk1.8.0_251jrelibjavaws.jar;C:Program FilesJavajdk1.8.0_251jrelibjce.jar;C:Program FilesJavajdk1.8.0_251jrelibjfr.jar;C:Program FilesJavajdk1.8.0_251jrelibjfxswt.jar;C:Program FilesJavajdk1.8.0_251jrelibjsse.jar;C:Program FilesJavajdk1.8.0_251jrelibmanagement-agent.jar;C:Program FilesJavajdk1.8.0_251jrelibplugin.jar;C:Program FilesJavajdk1.8.0_251jrelibresources.jar;C:Program FilesJavajdk1.8.0_251jrelibrt.jar;D:Techcodetech-jvmtargetclasses;C:Userslw.m2repositoryorgprojectlomboklombok1.18.22lombok-1.18.22.jar;C:Userslw.m2repositorycomfasterxmljacksoncorejackson-annotations2.12.4jackson-annotations-2.12.4.jar;C:Userslw.m2repositorycomfasterxmljacksoncorejackson-core2.12.4jackson-core-2.12.4.jar;C:Userslw.m2repositorycomfasterxmljacksoncorejackson-databind2.12.4jackson-databind-2.12.4.jar" com.tech.constantpool.Demo_6
0
Heap
 PSYoungGen      total 2560K, used 1851K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
  eden space 2048K, 90% used [0x00000000ffd00000,0x00000000ffeced48,0x00000000fff00000)
  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 3248K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 352K, capacity 388K, committed 512K, reserved 1048576K
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, avg   8.000
Number of entries       :     13382 =    321168 bytes, avg  24.000
Number of literals      :     13382 =    570928 bytes, avg  42.664
Total footprint         :           =   1052184 bytes
Average bucket size     :     0.669
Variance of bucket size :     0.670
Std. dev. of bucket size:     0.819
Maximum bucket size     :         6
StringTable statistics:
Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      1773 =     42552 bytes, avg  24.000
Number of literals      :      1773 =    158616 bytes, avg  89.462
Total footprint         :           =    681272 bytes
Average bucket size     :     0.030
Variance of bucket size :     0.030
Std. dev. of bucket size:     0.172
Maximum bucket size     :         3

Process finished with exit code 0

从上往下依次为：堆信息、符号表统计信息、串池统计信息，如果出现过gc，则最上面会打印gc信息。

此时并没有往串池添加10000个字符串，所以没有出现垃圾回收，串池中entries数量为1773

放开注释的代码，把10000个字符串加入到串池

for (int i = 0; i < 10000; i++) {
                String.valueOf(i).intern();
                ++c;
            }

打印日志：

"C:Program FilesJavajdk1.8.0_251binjava.exe" -Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc "-javaagent:D:TechresourceIntelliJ IDEA 2018.3.1libidea_rt.jar=64123:D:TechresourceIntelliJ IDEA 2018.3.1bin" -Dfile.encoding=UTF-8 -classpath "C:Program FilesJavajdk1.8.0_251jrelibcharsets.jar;C:Program FilesJavajdk1.8.0_251jrelibdeploy.jar;C:Program FilesJavajdk1.8.0_251jrelibextaccess-bridge-64.jar;C:Program FilesJavajdk1.8.0_251jrelibextcldrdata.jar;C:Program FilesJavajdk1.8.0_251jrelibextdnsns.jar;C:Program FilesJavajdk1.8.0_251jrelibextjaccess.jar;C:Program FilesJavajdk1.8.0_251jrelibextjfxrt.jar;C:Program FilesJavajdk1.8.0_251jrelibextlocaledata.jar;C:Program FilesJavajdk1.8.0_251jrelibextnashorn.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunec.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunjce_provider.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunmscapi.jar;C:Program FilesJavajdk1.8.0_251jrelibextsunpkcs11.jar;C:Program FilesJavajdk1.8.0_251jrelibextzipfs.jar;C:Program FilesJavajdk1.8.0_251jrelibjavaws.jar;C:Program FilesJavajdk1.8.0_251jrelibjce.jar;C:Program FilesJavajdk1.8.0_251jrelibjfr.jar;C:Program FilesJavajdk1.8.0_251jrelibjfxswt.jar;C:Program FilesJavajdk1.8.0_251jrelibjsse.jar;C:Program FilesJavajdk1.8.0_251jrelibmanagement-agent.jar;C:Program FilesJavajdk1.8.0_251jrelibplugin.jar;C:Program FilesJavajdk1.8.0_251jrelibresources.jar;C:Program FilesJavajdk1.8.0_251jrelibrt.jar;D:Techcodetech-jvmtargetclasses;C:Userslw.m2repositoryorgprojectlomboklombok1.18.22lombok-1.18.22.jar;C:Userslw.m2repositorycomfasterxmljacksoncorejackson-annotations2.12.4jackson-annotations-2.12.4.jar;C:Userslw.m2repositorycomfasterxmljacksoncorejackson-core2.12.4jackson-core-2.12.4.jar;C:Userslw.m2repositorycomfasterxmljacksoncorejackson-databind2.12.4jackson-databind-2.12.4.jar" com.tech.constantpool.Demo_6
[GC (Allocation Failure) [PSYoungGen: 2048K->488K(2560K)] 2048K->691K(9728K), 0.0008035 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
10000
Heap
 PSYoungGen      total 2560K, used 814K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
  eden space 2048K, 15% used [0x00000000ffd00000,0x00000000ffd519c0,0x00000000fff00000)
  from space 512K, 95% used [0x00000000fff00000,0x00000000fff7a020,0x00000000fff80000)
  to   space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
 ParOldGen       total 7168K, used 203K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
  object space 7168K, 2% used [0x00000000ff600000,0x00000000ff632d70,0x00000000ffd00000)
 metaspace       used 3279K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 357K, capacity 388K, committed 512K, reserved 1048576K
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, avg   8.000
Number of entries       :     13408 =    321792 bytes, avg  24.000
Number of literals      :     13408 =    571744 bytes, avg  42.642
Total footprint         :           =   1053624 bytes
Average bucket size     :     0.670
Variance of bucket size :     0.672
Std. dev. of bucket size:     0.819
Maximum bucket size     :         6
StringTable statistics:
Number of buckets       :     60013 =    480104 bytes, avg   8.000
Number of entries       :      6832 =    163968 bytes, avg  24.000
Number of literals      :      6832 =    401544 bytes, avg  58.774
Total footprint         :           =   1045616 bytes
Average bucket size     :     0.114
Variance of bucket size :     0.115
Std. dev. of bucket size:     0.340
Maximum bucket size     :         3

Process finished with exit code 0

出现了GC日志，entries的数量是6832个，说明串池中进行了垃圾回收，释放串池中一些没有引用的字符串。

StringTable串池垃圾回收

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