Android学习笔记六：SQLite使用

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一、新建一个MyDatabaseHelper继承SQLiteOpenHelper
public class MyDatabaseHelper extends SQLiteOpenHelper{    final String    CREATE_TABLE_SQL=            "Creat table dict(_id integer primary" +            "key autoincrement , word ,detail)";    public MyDatabaseHelper(Context context,String  name,int    version){        super(context,name,null,version);    }    @Override    public void onCreate(SQLiteDatabase db){        db.execSQL(CREATE_TABLE_SQL);    }    @Override    public void onUpgrade(SQLiteDatabase    db,int  oldverion,int   newverion){       System.out.println("onUpgrade Called"+oldverion+"----->"+"newberion");    }}
二、创建一个数据库
SQLiteOpenHelper    my_main_db=null;
my_main_db  =   new MyDatabaseHelper(this,"LowPowerWireless.db3",1);//路径在数据文件夹databases
三、如果要修改路径，请参考下面方法（没有验证）
最近碰到apk和后台的cpp code都需要访问一个数据库的问题。结果发现apk中创建的数据库外部的进程是没有权限去读/写的。这就需要把数据库文件创建到sdcard上。
后来发现在SQLiteOpenHelper(frameworks/base/core/java/android/database/sqlite /SQLiteOpenHelper.java)这个类中，创建数据库文件的路径是使用传入的contex的getDatabasePath获取的，这个 是不允许修改的（至少我没有找到）。
那我就仿照这个SQLiteOpenHelper写了一个abstract class SDSQLiteOpenHelper，其使用和SQLiteOpenHelper一样，然后只要加上相应的permission，这样就可以实现把数据库存储到sdcard上了。

import java.io.File;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.util.Log;
/**
* A helper class to manage database creation and version management.
* You create a subclass implementing {@link #onCreate}, {@link #onUpgrade} and
* optionally {@link #onOpen}, and this class takes care of opening the database
* if it exists, creating it if it does not, and upgrading it as necessary.
* Transactions are used to make sure the database is always in a sensible state.
* <p>For an example, see the NotePadProvider class in the NotePad sample application,
* in the <em>samples/</em> directory of the SDK.</p>
*/
public abstract class SDSQLiteOpenHelper {
    private static final String TAG = SDSQLiteOpenHelper.class.getSimpleName();
    private final Context mContext;
    private final String mName;
    private final CursorFactory mFactory;
    private final int mNewVersion;
    private SQLiteDatabase mDatabase = null;
    private boolean mIsInitializing = false;
    /**
     * Create a helper object to create, open, and/or manage a database.
     * The database is not actually created or opened until one of
     * {@link #getWritableDatabase} or {@link #getReadableDatabase} is called.
     *
     * @param context to use to open or create the database
     * @param name of the database file, or null for an in-memory database
     * @param factory to use for creating cursor objects, or null for the default
     * @param version number of the database (starting at 1); if the database is older,
     *     {@link #onUpgrade} will be used to upgrade the database
     */
    public SDSQLiteOpenHelper(Context context, String name, CursorFactory factory, int version) {
        if (version < 1) throw new IllegalArgumentException("Version must be >= 1, was " + version);
        mContext = context;
        mName = name;
        mFactory = factory;
        mNewVersion = version;
    }
    /**
     * Create and/or open a database that will be used for reading and writing.
     * Once opened successfully, the database is cached, so you can call this
     * method every time you need to write to the database.  Make sure to call
     * {@link #close} when you no longer need it.
     *
     * <p>Errors such as bad permissions or a full disk may cause this operation
     * to fail, but future attempts may succeed if the problem is fixed.</p>
     *
     * @throws SQLiteException if the database cannot be opened for writing
     * @return a read/write database object valid until {@link #close} is called
     */
    public synchronized SQLiteDatabase getWritableDatabase() {
        if (mDatabase != null && mDatabase.isOpen() && !mDatabase.isReadOnly()) {
            return mDatabase;  // The database is already open for business
        }
        if (mIsInitializing) {
            throw new IllegalStateException("getWritableDatabase called recursively");
        }
        // If we have a read-only database open, someone could be using it
        // (though they shouldn't), which would cause a lock to be held on
        // the file, and our attempts to open the database read-write would
        // fail waiting for the file lock.  To prevent that, we acquire the
        // lock on the read-only database, which shuts out other users.
        boolean success = false;
        SQLiteDatabase db = null;
        try {
            mIsInitializing = true;
            if (mName == null) {
                db = SQLiteDatabase.create(null);
            } else {
                    String path = getDatabasePath(mName).getPath();
                db = SQLiteDatabase.DatabopenOrCreatease(path, mFactory);
            }
            int version = db.getVersion();
            if (version != mNewVersion) {
                db.beginTransaction();
                try {
                    if (version == 0) {
                        onCreate(db);
                    } else {
                        onUpgrade(db, version, mNewVersion);
                    }
                    db.setVersion(mNewVersion);
                    db.setTransactionSuccessful();
                } finally {
                    db.endTransaction();
                }
            }
            onOpen(db);
            success = true;
            return db;
        } finally {
            mIsInitializing = false;
            if (success) {
                if (mDatabase != null) {
                    try { mDatabase.close(); } catch (Exception e) { }
                }
                mDatabase = db;
            } else {
                if (db != null) db.close();
            }
        }
    }
    /**
     * Create and/or open a database.  This will be the same object returned by
     * {@link #getWritableDatabase} unless some problem, such as a full disk,
     * requires the database to be opened read-only.  In that case, a read-only
     * database object will be returned.  If the problem is fixed, a future call
     * to {@link #getWritableDatabase} may succeed, in which case the read-only
     * database object will be closed and the read/write object will be returned
     * in the future.
     *
     * @throws SQLiteException if the database cannot be opened
     * @return a database object valid until {@link #getWritableDatabase}
     *     or {@link #close} is called.
     */
    public synchronized SQLiteDatabase getReadableDatabase() {
        if (mDatabase != null && mDatabase.isOpen()) {
            return mDatabase;  // The database is already open for business
        }
        if (mIsInitializing) {
            throw new IllegalStateException("getReadableDatabase called recursively");
        }
        try {
            return getWritableDatabase();
        } catch (SQLiteException e) {
            if (mName == null) throw e;  // Can't open a temp database read-only!
            Log.e(TAG, "Couldn't open " + mName + " for writing (will try read-only):", e);
        }
        SQLiteDatabase db = null;
        try {
            mIsInitializing = true;
            String path = getDatabasePath(mName).getPath();
            db = SQLiteDatabase.openDatabase(path, mFactory, SQLiteDatabase.OPEN_READWRITE);
            if (db.getVersion() != mNewVersion) {
                throw new SQLiteException("Can't upgrade read-only database from version " +
                        db.getVersion() + " to " + mNewVersion + ": " + path);
            }
            onOpen(db);
            Log.w(TAG, "Opened " + mName + " in read-only mode");
            mDatabase = db;
            return mDatabase;
        } finally {
            mIsInitializing = false;
            if (db != null && db != mDatabase) db.close();
        }
    }
    /**
     * Close any open database object.
     */
    public synchronized void close() {
        if (mIsInitializing) throw new IllegalStateException("Closed during initialization");
        if (mDatabase != null && mDatabase.isOpen()) {
            mDatabase.close();
            mDatabase = null;
        }
    }
   
    public File getDatabasePath(String name)
    {
            return new File("/sdcard/" + name);
    }
    /**
     * Called when the database is created for the first time. This is where the
     * creation of tables and the initial population of the tables should happen.
     *
     * @param db The database.
     */
    public abstract void onCreate(SQLiteDatabase db);
    /**
     * Called when the database needs to be upgraded. The implementation
     * should use this method to drop tables, add tables, or do anything else it
     * needs to upgrade to the new schema version.
     *
     * <p>The SQLite ALTER TABLE documentation can be found
     * <a href="http://sqlite.org/lang_altertable.html">here</a>. If you add new columns
     * you can use ALTER TABLE to insert them into a live table. If you rename or remove columns
     * you can use ALTER TABLE to rename the old table, then create the new table and then
     * populate the new table with the contents of the old table.
     *
     * @param db The database.
     * @param oldVersion The old database version.
     * @param newVersion The new database version.
     */
    public abstract void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion);
    /**
     * Called when the database has been opened.
     * Override method should check {@link SQLiteDatabase#isReadOnly} before
     * updating the database.
     *
     * @param db The database.
     */
    public void onOpen(SQLiteDatabase db) {}
}
看看是不是对你有用！！！
四、例子
我们大家都知道Android平台提供给我们一个数据库辅助类来创建或打开数据库，这个辅助类继承自SQLiteOpenHelper类，在该类的构造器中，调用Context中的方法创建并打开一个指定名称的数据库对象。继承和扩展SQLiteOpenHelper类主要做的工作就是重写以下两个方法。 

    onCreate(SQLiteDatabase db) : 当数据库被首次创建时执行该方法，一般将创建表等初始化操作在该方法中执行。 
    onUpgrade(SQLiteDatabse dv, int oldVersion,int new Version)：当打开数据库时传入的版本号与当前的版本号不同时会调用该方法。 
   
    除了上述两个必须要实现的方法外，还可以选择性地实现onOpen 方法，该方法会在每次打开数据库时被调用。 

  SQLiteOpenHelper 类的基本用法是：当需要创建或打开一个数据库并获得数据库对象时，首先根据指定的文件名创建一个辅助对象，然后调用该对象的getWritableDatabase 或 getReadableDatabase方法 获得SQLiteDatabase 对象。 

   调用getReadableDatabase 方法返回的并不总是只读数据库对象，一般来说该方法和getWriteableDatabase 方法的返回情况相同，只有在数据库仅开放只读权限或磁盘已满时才会返回一个只读的数据库对象。 
  
   下面通过一个简单的小例子说明SQLiteOpenDatabase的用法，其中包括创建数据库、插入数据、更新 、查询等等，我们将查询后获取到的数据显示到TextView上，看一下运行后的效果。 

 

MySQLiteHelper 
Java代码  
package xiaohang.zhimeng;  
  
import android.content.Context;  
import android.database.sqlite.SQLiteDatabase;  
import android.database.sqlite.SQLiteOpenHelper;  
import android.database.sqlite.SQLiteDatabase.CursorFactory;  
  
public class MySQLiteHelper extends SQLiteOpenHelper{  
    //调用父类构造器  
    public MySQLiteHelper(Context context, String name, CursorFactory factory,  
            int version) {  
        super(context, name, factory, version);  
    }  
  
    /** 
     * 当数据库首次创建时执行该方法，一般将创建表等初始化操作放在该方法中执行. 
     * 重写onCreate方法，调用execSQL方法创建表 
     * */  
    @Override  
    public void onCreate(SQLiteDatabase db) {  
        db.execSQL("create table if not exists hero_info("  
                  + "id integer primary key,"  
                  + "name varchar,"  
                  + "level integer)");  
          
    }  
      
    //当打开数据库时传入的版本号与当前的版本号不同时会调用该方法  
    @Override  
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {        
    }  
  
}  


Activity01 
Java代码  
package xiaohang.zhimeng;  
  
import android.app.Activity;  
import android.content.ContentValues;  
import android.database.Cursor;  
import android.database.sqlite.SQLiteDatabase;  
import android.graphics.Color;  
import android.os.Bundle;  
import android.widget.TextView;  
  
public class Activity01 extends Activity {  
    MySQLiteHelper myHelper;  
    TextView tv;  
      
    @Override  
    public void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
        setContentView(R.layout.main);  
        tv = (TextView)findViewById(R.id.tv);  
        //创建MySQLiteOpenHelper辅助类对象  
        myHelper = new MySQLiteHelper(this, "my.db", null, 1);  
        //向数据库中插入和更新数据  
        insertAndUpdateData(myHelper);  
        //查询数据  
        String result = queryData(myHelper);  
        tv.setTextColor(Color.RED);  
        tv.setTextSize(20.0f);  
        tv.setText("名字\t等级\n"+result);  
          
    }  
      
    //向数据库中插入和更新数据  
    public void insertAndUpdateData(MySQLiteHelper myHelper){  
        //获取数据库对象  
        SQLiteDatabase db = myHelper.getWritableDatabase();  
        //使用execSQL方法向表中插入数据  
        db.execSQL("insert into hero_info(name,level) values('bb',0)");  
        //使用insert方法向表中插入数据  
        ContentValues values = new ContentValues();  
        values.put("name", "xh");  
        values.put("level", 5);  
        //调用方法插入数据  
        db.insert("hero_info", "id", values);  
        //使用update方法更新表中的数据  
        //清空ContentValues对象  
        values.clear();  
        values.put("name", "xh");  
        values.put("level", 10);  
        //更新xh的level 为10  
        db.update("hero_info", values, "level = 5", null);  
        //关闭SQLiteDatabase对象  
        db.close();  
    }  
      
    //从数据库中查询数据  
    public String queryData(MySQLiteHelper myHelper){  
        String result = "";  
        //获得数据库对象  
        SQLiteDatabase db = myHelper.getReadableDatabase();  
        //查询表中的数据  
        Cursor cursor = db.query("hero_info", null, null, null, null, null, "id asc");  
        //获取name列的索引  
        int nameIndex = cursor.getColumnIndex("name");  
        //获取level列的索引  
        int levelIndex = cursor.getColumnIndex("level");  
        for (cursor.moveToFirst();!(cursor.isAfterLast());cursor.moveToNext()) {  
            result = result + cursor.getString(nameIndex)+ "\t\t";  
            result = result + cursor.getInt(levelIndex)+"       \n";  
        }  
        cursor.close();//关闭结果集  
        db.close();//关闭数据库对象  
        return result;  
    }  
      
    @Override  
    protected void onDestroy() {  
        SQLiteDatabase db = myHelper.getWritableDatabase();//获取数据库对象  
        //删除hero_info表中所有的数据 传入1 表示删除所有行------>点击back按钮  
        db.delete("hero_info", "1", null);  
        super.onDestroy();  
    }  
}  


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