新手必看，如何加载大图不 OOM

首先，我们试着往sdcard里放一张400k的图片，但是分辨率是2560*1600

布局简单

MainActivity

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {
    Button mbutton;
    ImageView mImageView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mbutton= (Button) findViewById(R.id.button);
        mImageView= (ImageView) findViewById(R.id.imageView);
        mbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
            Bitmap bitmap= BitmapFactory.decodeFile("/storage/sdcard/test.JPG");//将图片转换为bitmap
                mImageView.setImageBitmap(bitmap);
            }
        });
    }
}

很简单，设置点击按钮开始加载图片

结果：

报的错误：08-16 21:17:08.410 2947-2947/com.example.dell.myapplication E/AndroidRuntime: FATAL EXCEPTION: main                                                                             
java.lang.OutOfMemoryError                                                                                  
at android.graphics.BitmapFactory.nativeDecodeStream(Native Method)                                                                                  
at android.graphics.BitmapFactory.decodeStream(BitmapFactory.java:530)                                                                                  
at android.graphics.BitmapFactory.decodeFile(BitmapFactory.java:303)                                                                                  
at android.graphics.BitmapFactory.decodeFile(BitmapFactory.java:329)                                                                                  
at com.example.dell.myapplication.MainActivity$1.onClick(MainActivity.java:23)                                                                                  
at android.view.View.performClick(View.java:4240)                                                                                  
at android.view.View$PerformClick.run(View.java:17721)                                                                                  
at android.os.Handler.handleCallback(Handler.java:730)                                                                                  
at android.os.Handler.dispatchMessage(Handler.java:92)                                                                                  
at android.os.Looper.loop(Looper.java:137)                                                                                  
at android.app.ActivityThread.main(ActivityThread.java:5103)                                                                                  
at java.lang.reflect.Method.invokeNative(Native Method)                                                                                  
at java.lang.reflect.Method.invoke(Method.java:525)                                                                                  
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)                                                                                  
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)                                                                                  
at dalvik.system.NativeStart.main(Native Method)

这时候，你可能会想，天呐，我的图片明明只有400Kb啊，怎么可能会OutOfMemoryError！！是我的虚拟机太腊鸡了吗？

其实不是这样

在Android下采用ARGB来表示颜色  每个像素占4byte。

那么这张图片在Android虚拟机中实际占用内存：2560*1600*4byte=15.6M而我们的虚拟机创建时，Dalvik默认堆得大小是16M，所以这时基本就OOM了。（因为堆上还有别的东西）

这时候就要对我们的图片进行缩放

那么，如何缩放？当然是根据分辨率缩放。我们将图片和我们的虚拟机进行对比

宽缩放: 8

高缩放: 3

按照大的比例来缩放，这样缩放后的尺寸肯定宽高都小于等于手机的尺寸。

具体方法：

1 得到图片的分辨率

2 得到手机的分辨率

3缩放

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {
    Button mbutton;
    ImageView mImageView;
    int screenWidth;
    int screenHeight;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        WindowManager wm= (WindowManager) getSystemService(WINDOW_SERVICE);
       screenWidth= wm.getDefaultDisplay().getWidth();
        screenHeight=wm.getDefaultDisplay().getHeight();
    System.out.print("手机宽"+screenWidth+"手机高"+screenHeight);

        mbutton= (Button) findViewById(R.id.button);
        mImageView= (ImageView) findViewById(R.id.imageView);
        mbutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //创建bitmap工厂的配置参数
                BitmapFactory.Options options=new BitmapFactory.Options();
                //返回null，不去真正解析位图，只是得到宽高等信息
                options.inJustDecodeBounds=true;
               BitmapFactory.decodeFile("/storage/sdcard/test.JPG",options);
                int imgWidth=options.outWidth;
                int imgHeight=options.outHeight;
                System.out.print("图片宽"+imgWidth+"图片高"+imgHeight);

               // 计算缩放比
                int scale=1;
                int scalex=imgWidth/screenWidth;
                int scaley=imgHeight/screenHeight;
                scale=scalex>scaley?scalex:scaley;
                //按照缩放比显示图片
                options.inSampleSize=scale;
                //开始真正解析位图
                options.inJustDecodeBounds=false;
                Bitmap bitmap=BitmapFactory.decodeFile("/storage/sdcard/test.JPG",options);
                mImageView.setImageBitmap(bitmap);
            }
        });
    }
}

结果:

打印结果：

08-16 22:03:22.542 9790-9790/com.example.dell.myapplication I/System.out: 手机宽320手机高480
08-16 22:03:38.432 9790-9790/com.example.dell.myapplication I/System.out: 图片宽2560图片高1600