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               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   4        (object header: class)    0x01001200
 12   4    int FieldUpdaterTest.a        0
`Instance size: 16 bytes`
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

org.example.jucdemo2.atomic.AtomicTest22 object internals:
OFF  SZ                                        TYPE DESCRIPTION                  VALUE
  0   8                                             (object header: mark)        0x0000000000000001 (non-biasable; age: 0)
  8   4                                             (object header: class)       0x0101f0a8
 12   4                                         int AtomicTest22.a               0
 16   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。