java - 尝试打印字符串池信息时"statistics unavailable at this time"？

Question

我正在学习有关Java性能管理的类(class)，但是遇到一个问题，该问题使我无法为字符串池打印信息。这是我尝试向池中添加一千万个 String对象时得到的结果:

E:\IntelliJ Projects\ExploringStrings\src>java -XX:+PrintStringTableStatistics Main
Elapsed time was 28311 ms.
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, each 8
Number of entries       :     20527 =    492648 bytes, each 24
Number of literals      :     20527 =    776904 bytes, avg  37.848
Total footprint         :           =   1429640 bytes
Average bucket size     :     1.026
Variance of bucket size :     1.033
Std. dev. of bucket size:     1.017
Maximum bucket size     :         9
statistics unavailable at this moment

这是使用空主要方法运行程序时得到的结果:

E:\IntelliJ Projects\ExploringStrings\src>java -XX:+PrintStringTableStatistics Main
SymbolTable statistics:
Number of buckets       :     20011 =    160088 bytes, each 8
Number of entries       :     16615 =    398760 bytes, each 24
Number of literals      :     16615 =    608976 bytes, avg  36.652
Total footprint         :           =   1167824 bytes
Average bucket size     :     0.830
Variance of bucket size :     0.837
Std. dev. of bucket size:     0.915
Maximum bucket size     :         9
StringTable statistics:
Number of buckets       :     65536 =    524288 bytes, each 8
Number of entries       :      1734 =     27744 bytes, each 16
Number of literals      :      1734 =    112456 bytes, avg  64.854
Total footprsize_t         :           =    664488 bytes
Average bucket size     :     0.026
Variance of bucket size :     0.027
Std. dev. of bucket size:     0.163
Maximum bucket size     :         2

这是尝试向池中添加1000万个字符串的代码:

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        Date start = new Date();

        List<String> strings = new ArrayList<String>();

        for(Integer i = 1; i < 10000000; i++) {
            String s =  i.toString().intern(); //Adds new string object to the string pool
            strings.add(s);
        }

        Date end = new Date();
        System.out.println("Elapsed time was " + (end.getTime() - start.getTime()) + " ms.");

    }
}

因此，我的问题是:当我尝试添加1000万个对象时，为什么不能打印出有关字符串池的信息，而我的教练却可以呢？我无法理解其原因，因为这远远超出了我的Java编程水平。谢谢你们。

Answer 1

tl; dr
请在退出前等待几秒钟，以免出现“此时不可用的统计信息”问题。

Thread.sleep( Duration.ofSeconds( 5 ).toMillis() )

细节
如果您在控制台上引用该消息:

statistics unavailable at this moment

…我在使用与您的代码相似的代码时也遇到了同样的情况。

package work.basil.example;

import java.time.Duration;
import java.time.Instant;
import java.util.ArrayList;
import java.util.List;

public class Stringer
{
    public static void main ( String[] args )
    {
        Instant start = Instant.now();
        System.out.println( "Starting run at:" + start + ". Runtime.version(): " + Runtime.version() );
        List < String > strings = new ArrayList < String >();

        for ( int i = 1 ; i < 10_000_000 ; i++ )
        {
            String s = String.valueOf( i ).intern();  // Adds new string object to the string pool.
            strings.add( s );
        }

        System.out.println( "Elapsed time was " + Duration.between( start , Instant.now() ) );
    }
}

在AdoptOpenJDK上的具有Java 15的Intel上的macOS Mojave上运行时，在IntelliJ 2020.3.1中执行。

/Library/Java/JavaVirtualMachines/adoptopenjdk-15.jdk/Contents/Home/bin/java -XX:+PrintStringTableStatistics --enable-preview -javaagent:/Users/basilbourque/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/203.6682.168/IntelliJ IDEA 2020.3 EAP.app/Contents/lib/idea_rt.jar=52178:/Users/basilbourque/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/203.6682.168/IntelliJ IDEA 2020.3 EAP.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/basilbourque/IdeaProjects/ExampleJava/target/classes:/Users/basilbourque/.m2/repository/com/h2database/h2/1.4.200/h2-1.4.200.jar:/Users/basilbourque/.m2/repository/com/thedeanda/lorem/2.1/lorem-2.1.jar:/Users/basilbourque/.m2/repository/org/postgresql/postgresql/42.2.18/postgresql-42.2.18.jar:/Users/basilbourque/.m2/repository/org/checkerframework/checker-qual/3.5.0/checker-qual-3.5.0.jar:/Users/basilbourque/.m2/repository/org/apache/commons/commons-csv/1.8/commons-csv-1.8.jar work.basil.example.Stringer
Starting run at:2021-01-04T02:03:34.484633Z. Runtime.version(): 15.0.1+9
Elapsed time was PT6.219106S
SymbolTable statistics:
Number of buckets       :     32768 =    262144 bytes, each 8
Number of entries       :      2633 =     42128 bytes, each 16
Number of literals      :      2633 =     98200 bytes, avg  37.000
Total footprint         :           =    402472 bytes
Average bucket size     :     0.080
Variance of bucket size :     0.079
Std. dev. of bucket size:     0.280
Maximum bucket size     :         2
statistics unavailable at this moment

Process finished with exit code 0

…此时我得到的统计信息不可用。
因此，我大加猜测，我添加了一个Thread.sleep调用，等待5秒后退出。

try { Thread.sleep( Duration.ofSeconds( 5 ).toMillis() ); } catch ( InterruptedException e ) { e.printStackTrace(); }

因此，在运行此命令:

package work.basil.example;

import java.time.Duration;
import java.time.Instant;
import java.util.ArrayList;
import java.util.List;

public class Stringer
{
    public static void main ( String[] args )
    {
        Instant start = Instant.now();
        System.out.println( "Starting run at:" + start + ". Runtime.version(): " + Runtime.version() );
        List < String > strings = new ArrayList < String >();

        for ( int i = 1 ; i < 10_000_000 ; i++ )
        {
            String s = String.valueOf( i ).intern();  // Adds new string object to the string pool.
            strings.add( s );
        }

        System.out.println( "Elapsed time was " + Duration.between( start , Instant.now() ) );
        try { Thread.sleep( Duration.ofSeconds( 5 ).toMillis() ); } catch ( InterruptedException e ) { e.printStackTrace(); }
    }
}

我得到这个:

/Library/Java/JavaVirtualMachines/adoptopenjdk-15.jdk/Contents/Home/bin/java -XX:+PrintStringTableStatistics --enable-preview -javaagent:/Users/basilbourque/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/203.6682.168/IntelliJ IDEA 2020.3 EAP.app/Contents/lib/idea_rt.jar=52114:/Users/basilbourque/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/203.6682.168/IntelliJ IDEA 2020.3 EAP.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/basilbourque/IdeaProjects/ExampleJava/target/classes:/Users/basilbourque/.m2/repository/com/h2database/h2/1.4.200/h2-1.4.200.jar:/Users/basilbourque/.m2/repository/com/thedeanda/lorem/2.1/lorem-2.1.jar:/Users/basilbourque/.m2/repository/org/postgresql/postgresql/42.2.18/postgresql-42.2.18.jar:/Users/basilbourque/.m2/repository/org/checkerframework/checker-qual/3.5.0/checker-qual-3.5.0.jar:/Users/basilbourque/.m2/repository/org/apache/commons/commons-csv/1.8/commons-csv-1.8.jar work.basil.example.Stringer
Starting run at:2021-01-04T02:02:21.792377Z. Runtime.version(): 15.0.1+9
Elapsed time was PT5.887848S
SymbolTable statistics:
Number of buckets       :     32768 =    262144 bytes, each 8
Number of entries       :      2635 =     42160 bytes, each 16
Number of literals      :      2635 =     98280 bytes, avg  37.000
Total footprint         :           =    402584 bytes
Average bucket size     :     0.080
Variance of bucket size :     0.079
Std. dev. of bucket size:     0.281
Maximum bucket size     :         2
StringTable statistics:
Number of buckets       :   8388608 =  67108864 bytes, each 8
Number of entries       :  10000880 = 160014080 bytes, each 16
Number of literals      :  10000880 = 480062480 bytes, avg  48.000
Total footprint         :           = 707185424 bytes
Average bucket size     :     1.192
Variance of bucket size :     1.383
Std. dev. of bucket size:     1.176
Maximum bucket size     :         9

Process finished with exit code 0

等待退出后，statistics unavailable at this moment消息消失了，我们得到了完整的统计信息。
我们确实确实看到Number of literals从几千个跃升到一千万个以上。大概这是由于我们将一千万个数字的循环变成了字符串。
文档？
我试图对此进行研究，以了解PrintStringTableStatistics开关的已记录行为。但是我找不到任何文档。我现在不知道在哪里可以找到有关控制台工具java，javac等的文档。
如果有人有链接，请发表。