WeakHashMap JVM Java Example

WeakHashMapPoc.java

///usr/bin/env jbang "$0" "$@" ; exit $?

//JAVA_OPTIONS -Xms32m -Xmx32m -Xlog:gc* -XX:+UseSerialGC

/// -XX:+HeapDumpBeforeFullGC

/// jbang run -D=WEAK WeakHashMapPoc.java for infinite execution as weak references are collected before an OOO
/// jbang run -D=STRONG -D=WEAK WeakHashMapPoc.java OOO even using a WeakHashMap, as keys are strongly referenced
/// jbang run WeakHashMapPoc.java for an OOO as simple Map keys are strongly referenced

import java.nio.charset.StandardCharsets;
import java.util.Map;
import java.util.Random;
import java.util.WeakHashMap;
import java.util.HashMap;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;

public class WeakHashMapPoc {

  private static Function<Integer, String> getRandomString = (size) -> {
    byte[] array = new byte[size];
    new Random().nextBytes(array);	    
    return new String(array, StandardCharsets.UTF_8);
  };

  public static void main(String... args) {

    Map<String, String> map = null;

    if( System.getProperty("WEAK") != null)
      map = new WeakHashMap<String, String>();
    else
      map = new HashMap<String, String>();

    // strong references to the keys
    List<String> keys = new ArrayList<String>();

    while( Boolean.TRUE ) {
      String key = getRandomString.apply(1_000);
      map.put(key, getRandomString.apply(1_000));

      if( System.getProperty("STRONG") != null)
        // a strong reference to the key is being held here
        // the weak hash map will never free the memory
        // an OOO will eventually happen 
        keys.add( key );
      
      //System.out.println( map.size() ); // some debug output			
    }//while

  }//main

}//class