Java コードのパフォーマンスを向上させるためのさまざまなテクニックを共有する

/**
 - Testing String.intern.
 *
 - Run this class at least with -verbose:gc JVM parameter.
 */
public class InternTest {
    public static void main( String[] args ) {
        testStringPoolGarbageCollection();
        testLongLoop();
    }

    /**
     - Use this method to see where interned strings are stored
     - and how many of them can you fit for the given heap size.
     */
    private static void testLongLoop()
    {
        test( 1000 * 1000 * 1000 );
        //uncomment the following line to see the hand-written cache performance
        //testManual( 1000 * 1000 * 1000 );
    }

    /**
     - Use this method to check that not used interned strings are garbage collected.
     */
    private static void testStringPoolGarbageCollection()
    {
        //first method call - use it as a reference
        test( 1000 * 1000 );
        //we are going to clean the cache here.
        System.gc();
        //check the memory consumption and how long does it take to intern strings
        //in the second method call.
        test( 1000 * 1000 );
    }

    private static void test( final int cnt )
    {
        final List<String> lst = new ArrayList<String>( 100 );
        long start = System.currentTimeMillis();
        for ( int i = 0; i < cnt; ++i )
        {
            final String str = "Very long test string, which tells you about something " +
            "very-very important, definitely deserving to be interned #" + i;
//uncomment the following line to test dependency from string length
//            final String str = Integer.toString( i );
            lst.add( str.intern() );
            if ( i % 10000 == 0 )
            {
                System.out.println( i + "; time = " + ( System.currentTimeMillis() - start ) / 1000.0 + " sec" );
                start = System.currentTimeMillis();
            }
        }
        System.out.println( "Total length = " + lst.size() );
    }

    private static final WeakHashMap<String, WeakReference<String>> s_manualCache =
        new WeakHashMap<String, WeakReference<String>>( 100000 );

    private static String manualIntern( final String str )
    {
        final WeakReference<String> cached = s_manualCache.get( str );
        if ( cached != null )
        {
            final String value = cached.get();
            if ( value != null )
                return value;
        }
        s_manualCache.put( str, new WeakReference<String>( str ) );
        return str;
    }

    private static void testManual( final int cnt )
    {
        final List<String> lst = new ArrayList<String>( 100 );
        long start = System.currentTimeMillis();
        for ( int i = 0; i < cnt; ++i )
        {
            final String str = "Very long test string, which tells you about something " +
                "very-very important, definitely deserving to be interned #" + i;
            lst.add( manualIntern( str ) );
            if ( i % 10000 == 0 )
            {
                System.out.println( i + "; manual time = " + ( System.currentTimeMillis() - start ) / 1000.0 + " sec" );
                start = System.currentTimeMillis();
            }
        }
        System.out.println( "Total length = " + lst.size() );
    }
}

/**
 - Testing String.intern.
 *
 - Run this class at least with -verbose:gc JVM parameter.
 */
public class InternTest {
    public static void main( String[] args ) {
        testStringPoolGarbageCollection();
        testLongLoop();
    }

    /**
     - Use this method to see where interned strings are stored
     - and how many of them can you fit for the given heap size.
     */
    private static void testLongLoop()
    {
        test( 1000 * 1000 * 1000 );
        //uncomment the following line to see the hand-written cache performance
        //testManual( 1000 * 1000 * 1000 );
    }

    /**
     - Use this method to check that not used interned strings are garbage collected.
     */
    private static void testStringPoolGarbageCollection()
    {
        //first method call - use it as a reference
        test( 1000 * 1000 );
        //we are going to clean the cache here.
        System.gc();
        //check the memory consumption and how long does it take to intern strings
        //in the second method call.
        test( 1000 * 1000 );
    }

    private static void test( final int cnt )
    {
        final List<String> lst = new ArrayList<String>( 100 );
        long start = System.currentTimeMillis();
        for ( int i = 0; i < cnt; ++i )
        {
            final String str = "Very long test string, which tells you about something " +
            "very-very important, definitely deserving to be interned #" + i;
//uncomment the following line to test dependency from string length
//            final String str = Integer.toString( i );
            lst.add( str.intern() );
            if ( i % 10000 == 0 )
            {
                System.out.println( i + "; time = " + ( System.currentTimeMillis() - start ) / 1000.0 + " sec" );
                start = System.currentTimeMillis();
            }
        }
        System.out.println( "Total length = " + lst.size() );
    }

    private static final WeakHashMap<String, WeakReference<String>> s_manualCache =
        new WeakHashMap<String, WeakReference<String>>( 100000 );

    private static String manualIntern( final String str )
    {
        final WeakReference<String> cached = s_manualCache.get( str );
        if ( cached != null )
        {
            final String value = cached.get();
            if ( value != null )
                return value;
        }
        s_manualCache.put( str, new WeakReference<String>( str ) );
        return str;
    }

    private static void testManual( final int cnt )
    {
        final List<String> lst = new ArrayList<String>( 100 );
        long start = System.currentTimeMillis();
        for ( int i = 0; i < cnt; ++i )
        {
            final String str = "Very long test string, which tells you about something " +
                "very-very important, definitely deserving to be interned #" + i;
            lst.add( manualIntern( str ) );
            if ( i % 10000 == 0 )
            {
                System.out.println( i + "; manual time = " + ( System.currentTimeMillis() - start ) / 1000.0 + " sec" );
                start = System.currentTimeMillis();
            }
        }
        System.out.println( "Total length = " + lst.size() );
    }
}