android

Android内存管理

很多时候我们需要考虑Android平台上的内存管理问题，Dalvik VM给每个进程都分配了一定量的可用堆内存，当我们处理一些耗费资源的操作时可能会产生OOM错误(OutOfMemoryError)这样的异常，Android123观察了下国内的类似Market客户端设计，基本上都没有采用很好的内存管理机制和缓存处理。

在Java中内存管理，引用分为四大类，强引用HardReference、弱引用WeakReference、软引用SoftReference和虚引用PhantomReference。它们的区别也很明显，HardReference即正常实例化一个对象，即使虚拟机内存吃紧抛出OOM也不会导致这一引用的对象被回收；而WeakReference等更适合于一些数量不多，但体积稍微庞大的对象，在这四个引用中，它是最容易被垃圾回收的；而我们对于显示类似Android Market中每个应用的App Icon时可以考虑使用SoftReference来解决内存不至于快速回收，同时当内存短缺面临Java VM崩溃抛出OOM前时，软引用将会强制回收内存；最后的虚引用一般没有实际意义，仅仅观察GC的活动状态，对于测试比较实用同时必须和ReferenceQueue一起使用。

对于一组数据，我们可以通过HashMap的方式来添加一组SoftReference对象来临时保留一些数据，同时对于需要反复通过网络获取的不经常改变的内容，可以通过本地的文件系统或数据库来存储缓存。

WeakReference与SoftReference都可以用来保存对象的实例引用，这两个类与垃圾回收有关。

WeakReference是弱引用，其中保存的对象实例可以被GC回收掉。这个类通常用于在某处保存对象引用，而又不干扰该对象被GC回收，通常用于Debug、内存监视工具等程序中。因为这类程序一般要求即要观察到对象，又不能影响该对象正常的GC过程。

最近在JDK的Proxy类的实现代码中也发现了Weakrefrence的应用，Proxy会把动态生成的Class实例暂存于一个由Weakrefrence构成的Map中作为Cache。

SoftReference是软引用，它保存的对象实例，除非JVM即将OutOfMemory，否则不会被GC回收。这个特性使得它特别适合设计对象Cache。对于Cache，我们希望被缓存的对象最好始终常驻内存，但是如果JVM内存吃紧，为了不发生OutOfMemoryError导致系统崩溃，必要的时候也允许JVM回收Cache的内存，待后续合适的时机再把数据重新Load到Cache中。这样可以系统设计得更具弹性。

WeakReference的一个测试程序：

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import java.lang.ref.WeakReference;

public class WeakReferenceTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        A a = new A();
        a.str = "Hello, reference";
        WeakReference<A> weak = new WeakReference<A>(a);
        a = null;
        int i = 0;
        while (weak.get() != null) {
            System.out.println(String.format("Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i));
            if (i % 10 == 0) {
                System.gc();
                System.out.println("System.gc() was invoked!");
            }
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {

            }
        }
        System.out.println("object a was cleared by JVM!");
    }

}

程序运行结果：

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Get str from object of WeakReference: Hello, reference, count: 1
Get str from object of WeakReference: Hello, reference, count: 2
Get str from object of WeakReference: Hello, reference, count: 3
Get str from object of WeakReference: Hello, reference, count: 4
Get str from object of WeakReference: Hello, reference, count: 5
Get str from object of WeakReference: Hello, reference, count: 6
Get str from object of WeakReference: Hello, reference, count: 7
Get str from object of WeakReference: Hello, reference, count: 8
Get str from object of WeakReference: Hello, reference, count: 9
Get str from object of WeakReference: Hello, reference, count: 10
System.gc() was invoked!
object a was cleared by JVM!

SoftReference的一个测试程序：

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import java.lang.ref.SoftReference;

public class SoftReferenceTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        A a = new A();
        a.str = "Hello, reference";
        SoftReference<A> sr = new SoftReference<A>(a);
        a = null;
        int i = 0;
        while (sr.get() != null) {
            System.out.println(String.format("Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i));
            if (i % 10 == 0) {
                System.gc();
                System.out.println("System.gc() was invoked!");
            }
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {

            }
        }
        System.out.println("object a was cleared by JVM!");
    }

}

程序运行结果：

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Get str from object of SoftReference: Hello, reference, count: 1
Get str from object of SoftReference: Hello, reference, count: 2
Get str from object of SoftReference: Hello, reference, count: 3
Get str from object of SoftReference: Hello, reference, count: 4
Get str from object of SoftReference: Hello, reference, count: 5
Get str from object of SoftReference: Hello, reference, count: 6
Get str from object of SoftReference: Hello, reference, count: 7
Get str from object of SoftReference: Hello, reference, count: 8
Get str from object of SoftReference: Hello, reference, count: 9
Get str from object of SoftReference: Hello, reference, count: 10
System.gc() was invoked!
Get str from object of SoftReference: Hello, reference, count: 11
Get str from object of SoftReference: Hello, reference, count: 12
Get str from object of SoftReference: Hello, reference, count: 13
Get str from object of SoftReference: Hello, reference, count: 14
Get str from object of SoftReference: Hello, reference, count: 15
Get str from object of SoftReference: Hello, reference, count: 16
Get str from object of SoftReference: Hello, reference, count: 17
Get str from object of SoftReference: Hello, reference, count: 18
Get str from object of SoftReference: Hello, reference, count: 19
Get str from object of SoftReference: Hello, reference, count: 20
System.gc() was invoked!
Get str from object of SoftReference: Hello, reference, count: 21
Get str from object of SoftReference: Hello, reference, count: 22
Get str from object of SoftReference: Hello, reference, count: 23
Get str from object of SoftReference: Hello, reference, count: 24
Get str from object of SoftReference: Hello, reference, count: 25
Get str from object of SoftReference: Hello, reference, count: 26
Get str from object of SoftReference: Hello, reference, count: 27
Get str from object of SoftReference: Hello, reference, count: 28