笔记（十）——安卓存储知识

 /**
 * 获取外置TF卡路径/storage/sdcard1/或/storage/0F1C-240A/等
 * * @param mContext
 * @return
 */
 
 public static String getExtendedSDMemoryPath(Context mContext) {
 
 //ECOENDARY_STORAGE这个值，代表是第二储存，即为外置可移动SD卡。EXTERNAL_STORAGE则对应的是手机内部的存储。 StorageManager mStorageManager = (StorageManager) mContext.getSystemService(Context.STORAGE_SERVICE);
 
 Class<?> storageVolumeClazz = null;
 
 try {
 
 storageVolumeClazz = Class.forName("android.os.storage.StorageVolume");
 
 Method getVolumeList = mStorageManager.getClass().getMethod("getVolumeList");
 
 Method getPath = storageVolumeClazz.getMethod("getPath");
 
 Method isRemovable = storageVolumeClazz.getMethod("isRemovable");
 
 Object result = getVolumeList.invoke(mStorageManager);
 
 final int length = Array.getLength(result);
 
 for (int i = 0; i < length; i++) {
 
 Object storageVolumeElement = Array.get(result, i);
 
 String path = (String) getPath.invoke(storageVolumeElement);
 
 boolean removable = (Boolean) isRemovable.invoke(storageVolumeElement);
 
 if (removable) {
 
 return path + File.separator;
 
 } } }
 
 catch (ClassNotFoundException e) {
 
 e.printStackTrace();
 
 } catch (InvocationTargetException e) {
 
 e.printStackTrace();
 
 } catch (NoSuchMethodException e) {
 
 e.printStackTrace();
 
 } catch (IllegalAccessException e) { e.printStackTrace(); }
 
 return null;}

//数据库升级方案
public class DBHelper extends SQLiteOpenHelper {
 
    private static final String DATABASE_NAME = "student.db"; 
//数据库版本号
    private static final int DATABASE_VERSION = 1002;
 
    private static DBHelper instance = null;
  /*创建表语句 语句对大小写不敏感 create table 表名(字段名 类型，字段名 类型，…)*/
    private final String CREATE_PERSON = "create table Student (" +
            VALUE_ID + " integer primary key," +
            VALUE_NAME + " text ," +
            VALUE_ISBOY + " integer," +
            VALUE_AGE + " ingeter," +
            VALUE_ADDRESS + " text," +
            VALUE_PIC + " blob" +
            ")";
 
    public DBHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }
   public DBHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, name, factory, version);
       ///调用该方法，在onCreate()就不用调用onUpgrade(db, FIRST_DATABASE_VERSION, DATABASE_VERSION);
    }

    public synchronized static DBHelper getInstance(Context context) {
        if (instance == null) {
            instance = new DBHelper(context);
        }
        return instance;
    }
 
    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_PERSON);
         
        // 若不是第一个版本安装，直接执行数据库升级
        // 请不要修改FIRST_DATABASE_VERSION的值，其为第一个数据库版本大小，设置第一个数据库版本号
        final int FIRST_DATABASE_VERSION = 1000;
        onUpgrade(db, FIRST_DATABASE_VERSION, DATABASE_VERSION);
    }
 
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // 使用for实现跨版本升级数据库
        for (int i = oldVersion; i < newVersion; i++) {
            switch (i) {
            case 1000:
                upgradeToVersion1001(db);
                break;
            case 1001:
                upgradeToVersion1002(db);
                break;
            default:
                break;
            }
        }
    }
    //升级数据库表
    private void upgradeToVersion1001(SQLiteDatabase db){
        // student 表新增1个字段
        String sql1 = "ALTER TABLE Student ADD COLUMN age VARCHAR";
        db.execSQL(sql1);
    }
    //升级数据库表
    private void upgradeToVersion1002(SQLiteDatabase db){
        // student 表新增2个字段, 添加新字段只能一个字段一个字段加，sqlite有限制不予许一条语句加多个字段
        String sql1 = "ALTER TABLE Student ADD COLUMN tel VARCHAR";
        String sql2 = "ALTER TABLE Student ADD COLUMN address VARCHAR";
        db.execSQL(sql1);
        db.execSQL(sql2);
    }
}

 import java.lang.ref.SoftReference;
 
public class Main {
 
     public static void main(String[] args) {
 
         SoftReference<String> sr = new  SoftReference<String>(new  String("hello"));
 
         System.out.println(sr.get());
 
     }
 
 }

 import java.lang.ref.WeakReference;
 
 public class Main {
 
     public static void main(String[] args) {
 
         WeakReference<String> sr = new  WeakReference<String>(new  String("hello"));
 
         System.out.println(sr.get());
 
         System.gc();                //通知JVM的gc进行垃圾回收
 
         System.out.println(sr.get());
 
     }
 
 }

 Intent installIntent =new Intent(Intent.ACTION_VIEW); 
File apkPath =new File(Environment.getExternalStorageDirectory(),"appFile"); 
 File apkFile =newFile(apkPath,"myapp.apk");    
 
 if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
 
     Uri contentUri = FileProvider.getUriForFile(MainActivity.this,"com.example.myapp.fileprovider", apkFile);     installIntent.setDataAndType(contentUri,"application/vnd.android.package-archive");     installIntent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
 
 }else{
 
      installIntent.setDataAndType(Uri.fromFile(apkFile),"application/vnd.android.package-archive");
 
 }
 
     installIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
 
 if(installIntent.resolveActivity(getPackageManager()) !=null) { 
 
     startActivity(installIntent);
 
 }
 
 AndroidManifest.xml清单文件： 
 
 <provider android:name="android.support.v4.content.FileProvider"
 
     android:authorities="com.example.myapp.fileprovider"
 
     android:grantUriPermissions="true" android:exported="false">
 
     <meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/filepaths" />
 
 </provider> 
 
 filepaths .xml配置文件：
 
 <paths> <files-path path="images/" name="myimages" /></paths>