Java 性能优化手册 — 提高 Java 代码性能的各种技巧

0; time = 0.0 sec

50000; time = 0.03 sec

100000; time = 0.073 sec

150000; time = 0.13 sec

200000; time = 0.196 sec

250000; time = 0.279 sec

300000; time = 0.376 sec

350000; time = 0.471 sec

400000; time = 0.574 sec

450000; time = 0.666 sec

500000; time = 0.755 sec

550000; time = 0.854 sec

600000; time = 0.916 sec

650000; time = 1.006 sec

700000; time = 1.095 sec

750000; time = 1.273 sec

800000; time = 1.248 sec

850000; time = 1.446 sec

900000; time = 1.585 sec

950000; time = 1.635 sec

1000000; time = 1.913 sec

50000; time = 0.017 sec

100000; time = 0.009 sec

150000; time = 0.01 sec

200000; time = 0.009 sec

250000; time = 0.007 sec

300000; time = 0.008 sec

350000; time = 0.009 sec

400000; time = 0.009 sec

450000; time = 0.01 sec

500000; time = 0.013 sec

550000; time = 0.011 sec

600000; time = 0.012 sec

650000; time = 0.015 sec

700000; time = 0.015 sec

750000; time = 0.01 sec

800000; time = 0.01 sec

850000; time = 0.011 sec

900000; time = 0.011 sec

950000; time = 0.012 sec

1000000; time = 0.012 sec

2000000; time = 0.024 sec

3000000; time = 0.028 sec

4000000; time = 0.053 sec

5000000; time = 0.051 sec

6000000; time = 0.034 sec

7000000; time = 0.041 sec

8000000; time = 0.089 sec

9000000; time = 0.111 sec

10000000; time = 0.123 sec

1000000; time = 0.005 sec

2000000; time = 0.005 sec

3000000; time = 0.005 sec

4000000; time = 0.004 sec

5000000; time = 0.004 sec

6000000; time = 0.009 sec

7000000; time = 0.01 sec

8000000; time = 0.009 sec

9000000; time = 0.009 sec

10000000; time = 0.009 sec

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;

}

0; manual time = 0.001 sec

50000; manual time = 0.03 sec

100000; manual time = 0.034 sec

150000; manual time = 0.008 sec

200000; manual time = 0.019 sec

250000; manual time = 0.011 sec

300000; manual time = 0.011 sec

350000; manual time = 0.008 sec

400000; manual time = 0.027 sec

450000; manual time = 0.008 sec

500000; manual time = 0.009 sec

550000; manual time = 0.008 sec

600000; manual time = 0.008 sec

650000; manual time = 0.008 sec

700000; manual time = 0.008 sec

750000; manual time = 0.011 sec

800000; manual time = 0.007 sec

850000; manual time = 0.008 sec

900000; manual time = 0.008 sec

950000; manual time = 0.008 sec

1000000; manual time = 0.008 sec

50000; time = 0.019 sec

100000; time = 0.009 sec

150000; time = 0.009 sec

200000; time = 0.009 sec

250000; time = 0.009 sec

300000; time = 0.009 sec

350000; time = 0.011 sec

400000; time = 0.012 sec

450000; time = 0.01 sec

500000; time = 0.013 sec

550000; time = 0.013 sec

600000; time = 0.014 sec

650000; time = 0.018 sec

700000; time = 0.015 sec

750000; time = 0.029 sec

800000; time = 0.018 sec

850000; time = 0.02 sec

900000; time = 0.017 sec

950000; time = 0.018 sec

1000000; time = 0.021 sec

/**

- 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() );

}

}