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;
    }

}

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