Android Performance: Allocation Tracker

Author: 2016-07-14

Android

,

　　从Memory Monitor和Heap Viewer并不能看出占用内存的对象和分配内存在代码中发生的位置，为了进一步定位内存分配，需要使用Allocation Tracker。Allocation Tracker可以记录应用的内存分配，并列出所有分配的对象和调用栈。

　　下面例子使用的代码可以在这里找到，运行后点击“MEMORY CHURN”，如图1所示。

图1

图1

点击“DO INTERESTING THINGS WITH ARRAYS”后，会在主线程进行一系列耗时操作，动画会卡住，稍等片刻动画继续播放，表示耗时操作完成。

　　使用Memory Monitor和Heap Viewer观察按下“DO INTERESTING THINGS WITH ARRAYS”后的运行情况，如图2和图3。

图2

图2

图3

图3

从图2可见内存占用出现频繁快速地波动（Memory Churn），说明在短时间内申请了大量内存，然后很快被垃圾回收。从图3也可以看到垃圾回收（GCDaemon）频繁的执行。过于频繁的垃圾回收打断绘制，会导致界面卡顿（当然这里在主线程做耗时操作也会导致卡顿）。

　　下面使用Allocation Tracker定位频繁大量内存分配的位置。在Android Studio中点击底部的Android Monitor，打开Monitors选项卡。点击StartStart Allocation Tracking，然后点击应用的“DO INTERESTING THINGS WITH ARRAYS”按钮。等待耗时操作执行完毕，点击StopStop Allocation Tracking停止，Memory Monitor显示如图4所示。

图4

图4

　　接着Android Studio还会自动打开一个.alloc文件，在下拉菜单选择“Group by Method”，这里以调用栈的形式显示了各个方法中分配内存的次数和大小，按照Count和Size查找频繁大量分配内存的位置，定位到MemoryChurnActivity的imPrettySureSortingIsFree()方法，方法名后面的数字是行号，如图5。

图5

图5

　　在下拉菜单选择“Group by Allactor”，可以看到被频繁和大量分配的对象是AbstractStirngBuilder和String，如图6。

图6

图6

　　找到MemoryChurnActivity.java的imPrettySureSortingIsFree()：

/**
 * Sorts and prints every row of a 2D array, one element at a time.
 */
public void imPrettySureSortingIsFree() {
    // Throw together a nice, big 2D array of random numbers.
    int dimension = 300;
    int[][] lotsOfInts = new int[dimension][dimension];
    Random randomGenerator = new Random();
    for(int i = 0; i < lotsOfInts.length; i++) {
        for (int j = 0; j < lotsOfInts[i].length; j++) {
            lotsOfInts[i][j] = randomGenerator.nextInt();
        }
    }

    // Now go through and dump the sorted version of each row to output!
    for(int i = 0; i < lotsOfInts.length; i++) {
        String rowAsStr = "";
        for (int j = 0; j < lotsOfInts[i].length; j++) {
            // Clearly, the only reasonable way to construct a string is one character at a
            // time, with lots and lots of convenient concatenation.
            rowAsStr += getSorted(lotsOfInts[i])[j];
            if(j < (lotsOfInts[i].length - 1)){
                rowAsStr += ", ";
            }
        }
        Log.i("CachingActivityExercise", "Row " + i + ": " + rowAsStr);
    }
}

/**
 * Helper method, returns the sorted copy of an array.
 * @param input the unsorted array
 * @return the sorted array
 */
public int[] getSorted(int[] input) {
    int[] clone = input.clone();
    Arrays.sort(clone);
    return clone;
}

在rowAsStr += getSorted(lotsOfInts[i])[j]和rowAsStr += ", "两处在循环里直接拼接字符串导致了反复申请内存，当循环结束，反复申请的大量内存被回收，出现前面所见的Memory Churn现象。

　　使用StringBuilder替代字符串拼接，即可解决这一问题：

/**
 * Sorts and prints every row of a 2D array, one element at a time.
 */
public void imPrettySureSortingIsFree() {
    // Throw together a nice, big 2D array of random numbers.
    int dimension = 300;
    int[][] lotsOfInts = new int[dimension][dimension];
    Random randomGenerator = new Random();
    for(int i = 0; i < lotsOfInts.length; i++) {
        for (int j = 0; j < lotsOfInts[i].length; j++) {
            lotsOfInts[i][j] = randomGenerator.nextInt();
        }
    }

    // Now go through and dump the sorted version of each row to output! This time, use a
    // StringBuilder object so that we can construct one String per row, instead of wasting
    // String objects with ridiculous concatenation that never ends.
    // You may notice that the dancing pirate still seems to have lost the beat here. You would
    // be correct. What else could you do now to like that pirate keep his groove? What have you
    // learned?
    StringBuilder sb = new StringBuilder();
    String rowAsStr = "";
    for(int i = 0; i < lotsOfInts.length; i++) {
        sb.delete(0, rowAsStr.length()); // clear the previous row
        for (int j = 0; j < lotsOfInts[i].length; j++) {
            sb.append(getSorted(lotsOfInts[i])[j]);
            if(j < (lotsOfInts[i].length - 1)){
                sb.append(", ");
            }
        }
        rowAsStr = sb.toString();
        Log.i("CachingActivityExercise", "Row " + i + ": " + rowAsStr);
    }
}