1. 代码如下：
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;public class AtomicIntegerTest {final AtomicInteger startPosition = new AtomicInteger(0);final AtomicInteger wrotePosition = new AtomicInteger(0);final AtomicInteger committedPosition = new AtomicInteger(0);final AtomicInteger flushedPosition = new AtomicInteger(0);public static void main(String[] args) throws Exception {List<AtomicIntegerTest> list = new LinkedList<>();for (int i = 0; i < 1000000; i++) {list.add(new AtomicIntegerTest());}System.out.println("create instances 1000000");System.in.read();}
}
使用profiler 分析内存如下：
从上图可以看出来AtomicInteger 大概占用了64M。而AtomicIntegerTest对象实例整个占用了96M.
2. 修改之后代码如下：
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;public class AtomicIntegerFieldUpdaterTest {public static final AtomicIntegerFieldUpdater<AtomicIntegerFieldUpdaterTest> startPosition = AtomicIntegerFieldUpdater.newUpdater(AtomicIntegerFieldUpdaterTest.class,"startPositionInt");public static final AtomicIntegerFieldUpdater<AtomicIntegerFieldUpdaterTest> wrotePosition = AtomicIntegerFieldUpdater.newUpdater(AtomicIntegerFieldUpdaterTest.class,"wrotePositionInt");public static final AtomicIntegerFieldUpdater<AtomicIntegerFieldUpdaterTest> committedPosition = AtomicIntegerFieldUpdater.newUpdater(AtomicIntegerFieldUpdaterTest.class,"committedPositionInt");public static final AtomicIntegerFieldUpdater<AtomicIntegerFieldUpdaterTest> flushedPosition =AtomicIntegerFieldUpdater.newUpdater(AtomicIntegerFieldUpdaterTest.class,"flushedPositionInt");private volatile int startPositionInt = 0;private volatile int wrotePositionInt = 0;private volatile int committedPositionInt = 0;private volatile int flushedPositionInt = 0;public static void main(String[] args) throws Exception{List<AtomicIntegerFieldUpdaterTest> list = new LinkedList<>();for (int i = 0; i < 1000000; i++) {list.add(new AtomicIntegerFieldUpdaterTest());}System.out.println("create instances 1000000");System.in.read();}
}
使用profiler 分析内存如下：
AtomicIntegerFieldUpdaterTest整个对象大小的和为32M,相比之前的总共小了64M。大大的减少了内存的开销。
AtomicIntegerFieldUpdater如何使用
AtomicIntegerFieldUpdater必须是静态变量
被更新的变量必须被关键字volatile修饰
public class AtomicIntegerFieldUpdaterTest {public static final AtomicIntegerFieldUpdater<AtomicIntegerFieldUpdaterTest> startPosition = AtomicIntegerFieldUpdater.newUpdater(AtomicIntegerFieldUpdaterTest.class,"startPositionInt");private volatile int startPositionInt = 0;
}
4. 使用AtomicIntegerFieldUpdater后内存为什么会减少
代码如下：
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;class FieldUpdaterTest {public volatile int a = 0;private static final AtomicIntegerFieldUpdater updater = AtomicIntegerFieldUpdater.newUpdater(FieldUpdaterTest.class, "a");
}class AtomicTest22 {public volatile int a = 0;private AtomicInteger atomicInteger = new AtomicInteger(0);
}public class T {public static void main(String[] args) {FieldUpdaterTest test = new FieldUpdaterTest();System.out.println(ClassLayout.parseInstance(test).toPrintable());AtomicTest22 test2 = new AtomicTest22();System.out.println(ClassLayout.parseInstance(test2).toPrintable());}
}结果如下：
org.example.jucdemo2.atomic.FieldUpdaterTest object internals:
OFF SZ TYPE DESCRIPTION VALUE0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)8 4 (object header: class) 0x0100120012 4 int FieldUpdaterTest.a 0
`Instance size: 16 bytes`
Space losses: 0 bytes internal + 0 bytes external = 0 bytes totalorg.example.jucdemo2.atomic.AtomicTest22 object internals:
OFF SZ TYPE DESCRIPTION VALUE0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)8 4 (object header: class) 0x0101f0a812 4 int AtomicTest22.a 016 4 java.util.concurrent.atomic.AtomicInteger AtomicTest22.atomicInteger (object)20 4 (object alignment gap)
`Instance size: 24 bytes`
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
可以看到，FieldUpdaterTest比AtomicTest少了8 bytes。仔细分析上图结果，我们发现后者比前者多了4 bytes 大小的AtomicInteger对象，这也正是AtomicInteger中的成员变量int value，而AtomicIntegerFieldUpdater是staic final类型，即类变量，并不会占用当前对象的内存。正是基于AtomicIntegerFieldUpdater该使用特性，当字段所属的类会被创建大量的实例时，如果用AtomicInteger每个实例里面都要创建AtomicInteger对象，从而多出很多不必要的内存消耗。
