内存优化

获得手机内存方式一：

ActivityManager activityManager = (ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);

int memClass = activityManager.getMemoryClass();

int largeMemClass = activityManager.getLargeMemoryClass();

获得手机运行时内存情况：

float totalMemory = Runtime.getRuntime().totalMemory()/(1024*1024);

float freeMemory = Runtime.getRuntime().freeMemory()/(1024*1024);

float maxmemory = Runtime.getRuntime().maxMemory/（1024*1024）;

adb shell 进入设备底层

ps 查看所有进程

dumpsys meminfo +包名 查看具体进程的详细堆栈信息

一个app通常就是一个进程对应一个虚拟机

GC只在Heap剩余空间不足时才触发垃圾回收

GC触发时，所有线程都是会被暂停的

APP内存限制机制

每个app分配的最大内存限制，随不同设备而不同

吃内存的大户：图片

数据结构优化：

频繁的字符串拼接用StringBuilder

ArrayMap SparseArray替换HashMap

内存抖动（频繁创建变量，并很快弃之不用）会引起频繁GC

对象复用

复用系统自带的资源

ListView/GridView的ConvertView复用

避免在onDraw方法里面执行对象的创建

内存泄漏：由于代码瑕疵，导致这块内存虽然停止不用了，但依然被其他东西引用这，是的GC没法对他回收，

内存泄漏会导致剩余可用Heap越来越少，频繁GC

用Application Context而不是Activity Context

注意Cursor对象是否及时关闭

OOM问题优化方法

注意临时BItmap对象的及时回收

避免Bitmap的浪费

加载Bitmap:缩放比例，解码格式，局部加载

示例如下：

package com.example.admin.myapplication.oom;import android.content.Intent;import android.database.Cursor;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.graphics.BitmapRegionDecoder;import android.graphics.Rect;import android.net.Uri;import android.os.Bundle;import android.provider.MediaStore;import android.support.v7.app.AppCompatActivity;import android.util.DisplayMetrics;import android.util.Log;import android.view.View;import android.widget.Button;import android.widget.ImageView;import com.example.admin.myapplication.R;import java.io.File;import java.io.FileInputStream;import java.io.FileNotFoundException;public class OOMActivity extends AppCompatActivity {    private ImageView img;    private Bitmap bitmap;    private File file = null;    private int SCREEN_WIDTH, SCREEN_HEIGHT;    private int shifpx = 0;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main2);         img = (ImageView) findViewById(R.id.img);        DisplayMetrics dm = new DisplayMetrics();        dm = this.getResources().getDisplayMetrics();        SCREEN_WIDTH = dm.widthPixels;        SCREEN_HEIGHT = dm.heightPixels;        Log.e("oomactivity","screen_width:"+SCREEN_WIDTH);        Button button = (Button) findViewById(R.id.choosepic);        button.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View view) {                getPic();            }        });        findViewById(R.id.picpoti).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View view) {                changePicOopti();            }        });        findViewById(R.id.changergb).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View view) {                changeRGB();            }        });        findViewById(R.id.partload).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View view) {                partLoad();            }        });        findViewById(R.id.movepart).setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View view) {                shifpx++;                partLoad();            }        });    }    private void getPic(){        Intent intent = new Intent(Intent.ACTION_PICK,                android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);        startActivityForResult(intent, 1);//        String path = "/storage/emulated/0/qingming.jpg";//        bitmap = BitmapFactory.decodeResource(getResources(),R.drawable.qingming);//        img.setImageBitmap(bitmap);//        Log.e("oomActivity","bitmap.length:"+bitmap.getByteCount());    }    @Override    protected void onActivityResult(int requestCode, int resultCode, Intent data) {        try{            String path = getRealPathFromURI(data.getData());            file = new File(path);            if (file == null) return;            if (file.length()==0){                file .delete();                return;            }            Log.e("oomActivity","file;"+file.getName()+" ,length:"+file.length());            FileInputStream fin = new FileInputStream(file);            bitmap = BitmapFactory.decodeStream(fin);            Log.e("oomActivity","bitmap.lenght:"+bitmap.getByteCount());            Bitmap bitmap1 = BitmapFactory.decodeFile(path);            Log.e("oomActivity","bitmap1.lenght:+"+bitmap1.getByteCount());            img.setImageBitmap(bitmap);        }catch(Exception e){            e.printStackTrace();            Log.e("oomActivity",e.toString());        }    }    /**     *缩放     */    private void changePicOopti(){        Log.e("oomActivity","---------------------------------------------");        if (file==null){            return;        }        try{            BitmapFactory.Options options= new BitmapFactory.Options();            options.inJustDecodeBounds = true;            BitmapFactory.decodeStream(new FileInputStream(file),null,options);            int width_tem = options.outWidth,height_tmp = options.outHeight;            int scale = 2;            while(true){                if (width_tem/scale<SCREEN_WIDTH){                    break;                }                scale+=2;            }            scale=4;            BitmapFactory.Options options1 = new BitmapFactory.Options();            options1.inSampleSize= scale;            Log.e("oomActivity","bitmap.scale:"+scale);            FileInputStream fileInputStream = new FileInputStream(file);            bitmap = BitmapFactory.decodeStream(fileInputStream,null,options1);            Log.e("oomActivity","scale_bitmap.length:"+bitmap.getByteCount());            img.setImageBitmap(bitmap);        }catch (Exception e){            e.printStackTrace();        }    }    /**     * 改变RGB     */    private void changeRGB(){        Log.e("oomActivity","---------------------------------------------");        if (file != null){            BitmapFactory.Options options = new BitmapFactory.Options();            options.inPreferredConfig =Bitmap.Config.RGB_565;            try {                FileInputStream fileInputStream = new FileInputStream(file);                bitmap = BitmapFactory.decodeStream(fileInputStream,null,options);                Log.e("oomActivity","RGB+565_bitmap.length"+ bitmap.getByteCount());                img.setImageBitmap(bitmap);            } catch (FileNotFoundException e) {                e.printStackTrace();                Log.e("oomActivity",""+e.toString());            }        }    }    /**     * 局部显示     */    private void partLoad(){        if (file==null)            return;        try{            FileInputStream fileInputStream = new FileInputStream(file);            BitmapFactory.Options options = new BitmapFactory.Options();            BitmapFactory.decodeStream(fileInputStream,null,options);            int widht = options.outWidth;            int height = options.outHeight;            //设置显示图片的中心区域            fileInputStream = new FileInputStream(file);            BitmapRegionDecoder bitmapRegionDecoder = BitmapRegionDecoder.newInstance(fileInputStream,false);            BitmapFactory.Options options1 = new BitmapFactory.Options();            bitmap = bitmapRegionDecoder.decodeRegion(new Rect(widht/2-SCREEN_WIDTH/2+shifpx,height/2-SCREEN_HEIGHT/2,widht/2+SCREEN_WIDTH/2+shifpx,height/2+SCREEN_HEIGHT/2),options);            img.setImageBitmap(bitmap);        }catch(Exception e){        }    }    public String getRealPathFromURI(Uri contentUri){        String res = null;        String proj = (MediaStore.Images.Media.DATA);        Cursor cursor = this.getContentResolver().query(contentUri, new String[]{proj},null,null,null);        if (cursor.moveToFirst()){            int column_index = cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);            res = cursor.getString((column_index));        }        cursor.close();        return res;    }}