"statistics unavailable at this time" when trying to print String Pool information?

tl;dr

Wait a few seconds before exiting, to avoid "statistics unavailable at this moment" issue.

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

Details

If you are referring to that message on the console:

statistics unavailable at this moment

… I experienced the same with code similar to yours.

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() ) );
    }
}

When run on macOS Mojave on Intel with Java 15 from AdoptOpenJDK, executed within IntelliJ 2020.3.1.

/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

…where I get the same statistics unavailable at this moment message.

So on a wild guess, I added a Thread.sleep call to wait 5 seconds before exiting.

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

So, in running this:

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(); }
    }
}

I get this:

/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

After waiting to exit, the statistics unavailable at this moment message is gone, and we get full statistics.

We do indeed see Number of literals jump from a few thousand to over ten million. Presumably this is due to our loop of ten million numbers turned into strings.

Documentation?

I tried to research this, to learn the documented behavior of the PrintStringTableStatistics switch. But I cannot find any documentation. I have no idea where to find documentation nowadays for the console tools java, javac, and so on.

If anyone has a link, please post.


As an addendum, that line simply says that some thread that wants to request statistics could not acquire the lock, as seen here.

The method that is trying to be called (try_resize_lock) is here. It surely looks like the thread that requested the statistics could not acquire a certain lock (I guess, it's because some other thread was updating the ConcurrentHashTable in that exact moment).

I have not looked at the exact details and implementation of why this thread can no simply wait until it could get the lock (like synchronized does), but I guess there are compelling reasons that I am not aware of.

Tags:

Java

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