Android面试题----Android为每个应用分配多少内存？

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熟悉Android内存分配机制的朋友都知道，Android为每个进程分配内存时，采用弹性的分配方式，即刚开始并不会给应用分配很多的内存，而是给每一个进程分配一个“够用”的内存大小。

那Android到底为每个应用分配多少内存呢？我们可以实际测试一下：

以本人手上的努比亚NX510J手机为例：

private void getMaxMemoryInfo(){
Runtime rt = Runtime.getRuntime();
long maxMemory = rt.maxMemory();
Log.e("MaxMemory:", Long.toString(maxMemory/(1024*1024)));
ActivityManager activityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
Log.e("MemoryClass:", Long.toString(activityManager.getMemoryClass()));
Log.e("LargeMemoryClass:", Long.toString(activityManager.getLargeMemoryClass()));
}

输出结果为：

06-06 15:27:22.740 11917-11917/com.suning.myapp E/MaxMemory:: 192
06-06 15:27:22.740 11917-11917/com.suning.myapp E/MemoryClass:: 192
06-06 15:27:22.740 11917-11917/com.suning.myapp E/LargeMemoryClass:: 512

把AndroidManifest.xml中的application标签加上

<application
...
android:largeHeap="true"
...>
...
</application>

输出结果为：

06-06 15:32:06.168 21973-21973/com.suningtang.myapp E/MaxMemory:: 512
06-06 15:32:06.168 21973-21973/com.suningtang.myapp E/MemoryClass:: 192
06-06 15:32:06.168 21973-21973/com.suningtang.myapp E/LargeMemoryClass:: 512

可以看到，设置largeHeap为true时， 通过rt.maxMemory();获取的值为512M。
因此，对于本人这台手机，系统正常分配的内存最多为192M；当设置largeHeap时，最多可申请512M。当超过这个值时，就会出现OOM了。
这个值是在哪设置的呢？查看/system/build.prop文件内容：

shell@NX510J:/ $ cat /system/build.prop | grep heap
dalvik.vm.heapsize=36m

dalvik.vm.heapstartsize=8m    ----起始分配内存
dalvik.vm.heapgrowthlimit=192m ---- 一般情况app申请的最大内存 dalvik.vm.heapsize=512m   ---- 设置largeheap时，App可用的最大内存dalvik.vm.heaptargetutilization=0.75  ---- GC相关
dalvik.vm.heapminfree=512k
dalvik.vm.heapmaxfree=8m     ----- GC机制相关

上面的标注了各参数的含义。
那ActivityManager的getMemoryClass()和getLargeMemoryClass()方法返回的的是哪里的值呢？
//ActivityManager.java

public int getMemoryClass() {
return staticGetMemoryClass();
}

/** @hide */
static public int staticGetMemoryClass() {

String vmHeapSize = SystemProperties.get("dalvik.vm.heapgrowthlimit", "");
if (vmHeapSize != null && !"".equals(vmHeapSize)) {
return Integer.parseInt(vmHeapSize.substring(0, vmHeapSize.length()-1));
}
return staticGetLargeMemoryClass();
}

/**
* Return the approximate per-application memory class of the current
* device when an application is running with a large heap.  This is the
* space available for memory-intensive applications; most applications
* should not need this amount of memory, and should instead stay with the
* {@link #getMemoryClass()} limit.  The returned value is in megabytes.
* This may be the same size as {@link #getMemoryClass()} on memory
* constrained devices, or it may be significantly larger on devices with
* a large amount of available RAM.
*
* <p>The is the size of the application's Dalvik heap if it has
* specified <code>android:largeHeap="true"</code> in its manifest.
*/
public int getLargeMemoryClass() {
return staticGetLargeMemoryClass();
}

/** @hide */
static public int staticGetLargeMemoryClass() {
String vmHeapSize = SystemProperties.get("dalvik.vm.heapsize", "16m");
return Integer.parseInt(vmHeapSize.substring(0, vmHeapSize.length() - 1));
}

上面的源码表明，getMemoryClass()和getLargeMemoryClass()方法最终读取的仍然是dalvik.vm.heapgrowthlimit和dalvik.vm.heapsize的值。而且，dalvik.vm.heapsize默认值为16M，这也是解释了google的原生OS默认值是16M了。而dalvik.vm.heapgrowthlimit和dalvik.vm.heapsize的值各个手机厂家的OS会对这个值进行修改，所以存在差异。

在App中获取内存信息

我们在应用中可以通过ActivityManager的MemoryInfo内部类获取内存信息，方法如下：

private void getMemoryInfo() {
ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
ActivityManager.MemoryInfo info = new ActivityManager.MemoryInfo();
manager.getMemoryInfo(info);
Log.e("Memory","系统总内存:"+(info.totalMem / (1024*1024))+"M");
Log.e("Memory","系统剩余内存:"+(info.availMem / (1024*1024))+"M");
Log.e("Memory","系统是否处于低内存运行："+info.lowMemory );
Log.e("Memory","系统剩余内存低于"+( info.threshold  / (1024*1024))+"M时为低内存运行");
}

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