Android 创建SQLite数据库（一）

　　Android内置了轻量级的数据库SQLite，这里将自己理解作个记录，方便自己复习。

　　一.首先，创建SQLite数据库比较常见的方式是通过Android提供的SQLiteOpenHelper来实现，先贴一段代码：

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import java.io.File;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    private static final String TAG = "CustomSQLiteOpenHelper";
    private Button mButton;
    private CustomSQLiteOpenHelper customSQLiteOpenHelper;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mButton = (Button)findViewById(R.id.btn1);
        mButton.setOnClickListener(this);
        customSQLiteOpenHelper = new CustomSQLiteOpenHelper(this);//创建SQLIteOpenHelper对象（1）
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.btn1:
                customSQLiteOpenHelper.getWritableDatabase();//通过getWritableDatabase()方式来新建SQLite数据库（2）
                break;
            default:
                break;
        }
    }

    class CustomSQLiteOpenHelper extends SQLiteOpenHelper {
        private static final String DATABASE_NAME = "book_store.db";//数据库名字
        private static final int DATABASE_VERSION = 1;//数据库版本号
        private static final String CREATE_TABLE = "create table bookStore ("
                + "id integer primary key autoincrement,"
                + "book_name text, "
                + "author text, "
                + "price real)";//数据库里的表

        public CustomSQLiteOpenHelper(Context context) {
            this(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        private CustomSQLiteOpenHelper(Context context, String name, CursorFactory factory, int version) {
            super(context, name, factory, version);//调用到SQLiteOpenHelper中
            Log.d(TAG,"New CustomSQLiteOpenHelper");
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            Log.d(TAG,"onCreate");
            db.execSQL(CREATE_TABLE);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        }
    }
}

　　在MainActivity布局文件activity_main.xml里，只有一个Button控件，点击该Button，就会通过调用SQLiteOpenHelper.getWritableDatabase()的方式来创建名为DATABASE_NAME的数据库。数据库是否新建成功，可以在/data/data/<Package_Name>/databse/目录下确认。

　　用于创建SQLite数据库比较重要的代码：（1）建立SQLiteOpenHelper对象；（2）调用getWritableDatabase（）来建立SQLite数据库。下面来看看这两句代码主要做了什么。

　　二.代码分析

　　（1）建立SQLiteOpenHelper对象

    /**
     * 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.*/
    public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version,
            DatabaseErrorHandler errorHandler) {
        if (version < 1) throw new IllegalArgumentException("Version must be >= 1, was " + version);

        mContext = context;
        mName = name;
        mFactory = factory;
        mNewVersion = version;
        mErrorHandler = errorHandler;
    }

　　从注释中就可以看出，该构造函数只是建立SQLiteOpenHelper对象，以及进行了一些变量的初始化动作，所以正真创建SQLite数据库地方不是在这里，而是在getWritableDatabase() 或者 getReadableDatabase()中实现的。接下来看这两个函数是如何创建SQLite数据库的。

　　（2）调用getWritableDatabase（）函数

    /**
     * Create and/or open a database that will be used for reading and writing.
     * The first time this is called, the database will be opened and
     * {@link #onCreate}, {@link #onUpgrade} and/or {@link #onOpen} will be
     * called.
     *
     * <p>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 the database.)
     * Errors such as bad permissions or a full disk may cause this method
     * to fail, but future attempts may succeed if the problem is fixed.</p>
     *
     * <p class="caution">Database upgrade may take a long time, you
     * should not call this method from the application main thread, including
     * from {@link android.content.ContentProvider#onCreate ContentProvider.onCreate()}.
     **/
    public SQLiteDatabase getWritableDatabase() {
        synchronized (this) {
            return getDatabaseLocked(true);
        }
    }

    /**
     * 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.
     *
     * <p class="caution">Like {@link #getWritableDatabase}, this method may
     * take a long time to return, so you should not call it from the
     * application main thread, including from
     * {@link android.content.ContentProvider#onCreate ContentProvider.onCreate()}.
     **/
    public SQLiteDatabase getReadableDatabase() {
        synchronized (this) {
            return getDatabaseLocked(false);
        }
    }

　　getWritableDatabase() & getReadableDatabase()函数中都是调用getDatabaseLocked()函数，仅仅是传的参数不同，这两个函数的功能基本上是相同的。注释中也举例说明了当磁盘满了是两者间的区别。下面接着看getDatabaseLocked()中的内容。

    private SQLiteDatabase getDatabaseLocked(boolean writable) {
        if (mDatabase != null) {//第一次进来时，mDatabase为空
            if (!mDatabase.isOpen()) {
                // Darn!  The user closed the database by calling mDatabase.close().
                mDatabase = null;//当mDatabase被close后，再次打开时需要重新创建
            } else if (!writable || !mDatabase.isReadOnly()) {
                // The database is already open for business.
                return mDatabase;//直接返回当前mDatabase
            }
        }

        if (mIsInitializing) {
            throw new IllegalStateException("getDatabase called recursively");
        }

        SQLiteDatabase db = mDatabase;
        try {
            mIsInitializing = true;

            if (db != null) {
                if (writable && db.isReadOnly()) {
                    //数据库从ReadOnly变为ReadWrite时调用
                    db.reopenReadWrite();
                }
            } else if (mName == null) {
                db = SQLiteDatabase.create(null);
            } else {
                try {
                    //DEBUG_STRICT_READONLY 默认为FALSE，因为实际应用过程中，只读数据库不实用，自己Debug可以设置为TRUE测试。
                    if (DEBUG_STRICT_READONLY && !writable) {
                        final String path = mContext.getDatabasePath(mName).getPath();
                        db = SQLiteDatabase.openDatabase(path, mFactory,
                                SQLiteDatabase.OPEN_READONLY, mErrorHandler);
                    } else {
                        db = mContext.openOrCreateDatabase(mName, mEnableWriteAheadLogging ?
                                Context.MODE_ENABLE_WRITE_AHEAD_LOGGING : 0,
                                mFactory, mErrorHandler);
                    }
                } catch (SQLiteException ex) {
                    if (writable) {
                        throw ex;
                    }
                    Log.e(TAG, "Couldn't open " + mName
                            + " for writing (will try read-only):", ex);
                    final String path = mContext.getDatabasePath(mName).getPath();
                    db = SQLiteDatabase.openDatabase(path, mFactory,
                            SQLiteDatabase.OPEN_READONLY, mErrorHandler);
                }
            }

            onConfigure(db);
            //下面数据库版本信息发生变化时调用
            final int version = db.getVersion();//第一次创建，version值为0
            if (version != mNewVersion) {
                if (db.isReadOnly()) {
                    throw new SQLiteException("Can't upgrade read-only database from version " +
                            db.getVersion() + " to " + mNewVersion + ": " + mName);
                }

                db.beginTransaction();
                try {
                    if (version == 0) {
                        onCreate(db);//该函数为抽象函数，可以在重载该函数时执行创建数据库命令
                    } else {
                        if (version > mNewVersion) {
                            //版本降低时调用，但是默认不支持该操作，会报“SQLiteException：Can't downgrade database from version”
                            onDowngrade(db, version, mNewVersion);
                        } else {
                            //该函数为抽象函数，可以添加自己想实现的效果代码
                            onUpgrade(db, version, mNewVersion);
                        }
                    }
                    db.setVersion(mNewVersion);
                    db.setTransactionSuccessful();
                } finally {
                    db.endTransaction();
                }
            }

            onOpen(db);

            if (db.isReadOnly()) {
                Log.w(TAG, "Opened " + mName + " in read-only mode");
            }

            mDatabase = db;
            return db;
        } finally {
            mIsInitializing = false;
            if (db != null && db != mDatabase) {
                db.close();
            }
        }
    }

　　其中DEBUG_STRICT_READONLY变量默认为False，所以正常建立的数据库都是Readable & Writable。所以上面新建数据库代码主要可以分为三个部分：

　　1.如果mDataBase不为空，并且处于打开状态时，直接返回，所以多次调用getWritableDatabase()或getReadableDatabase()和只调用一次的效果是一样的。

　　2.如果mDataBase为空，则调用mContext.openOrCreateDatabase()来创建数据库。

　　3.当数据库版本信息发生变化时，做相应的升/降版本处理。

　　那么mContext.openOrCreateDatabase()函数里又做了哪些事情呢？进入到ContextImpl.java文件中一探究竟。

    @Override
    public SQLiteDatabase openOrCreateDatabase(String name, int mode, CursorFactory factory,
            DatabaseErrorHandler errorHandler) {
        File f = validateFilePath(name, true);//判断数据库name是否有效
        int flags = SQLiteDatabase.CREATE_IF_NECESSARY;
        if ((mode & MODE_ENABLE_WRITE_AHEAD_LOGGING) != 0) {
            flags |= SQLiteDatabase.ENABLE_WRITE_AHEAD_LOGGING;
        }
　　　　　//openDatabase
        SQLiteDatabase db = SQLiteDatabase.openDatabase(f.getPath(), factory, flags, errorHandler);
        setFilePermissionsFromMode(f.getPath(), mode, 0);//设置文件读写权限Mode
        return db;
    }

    private File validateFilePath(String name, boolean createDirectory) {
        File dir;
        File f;

        if (name.charAt(0) == File.separatorChar) {
　　　　　　　//可以自己定义数据库所在目录，但是要注意目录权限，比如以User身份往System所属的目录里添加文件
            String dirPath = name.substring(0, name.lastIndexOf(File.separatorChar));
            dir = new File(dirPath);
            name = name.substring(name.lastIndexOf(File.separatorChar));
            f = new File(dir, name);
        } else {
            dir = getDatabasesDir();
            f = makeFilename(dir, name);
        }

        if (createDirectory && !dir.isDirectory() && dir.mkdir()) {
            FileUtils.setPermissions(dir.getPath(),
                FileUtils.S_IRWXU|FileUtils.S_IRWXG|FileUtils.S_IXOTH,
                -1, -1);
        }

        return f;
    }

    private File getDatabasesDir() {
        synchronized (mSync) {
            if (mDatabasesDir == null) {
                //这个得到的结果就是/data/data/<PackageName>/database
                mDatabasesDir = new File(getDataDirFile(), "databases");
            }
            if (mDatabasesDir.getPath().equals("databases")) {
                mDatabasesDir = new File("/data/system");
            }
            return mDatabasesDir;
        }
    }

    @SuppressWarnings("deprecation")
    static void setFilePermissionsFromMode(String name, int mode,
            int extraPermissions) {
        int perms = FileUtils.S_IRUSR|FileUtils.S_IWUSR
            |FileUtils.S_IRGRP|FileUtils.S_IWGRP
            |extraPermissions;//默认权限为-rx-rx---
        if ((mode&MODE_WORLD_READABLE) != 0) {
            perms |= FileUtils.S_IROTH;
        }
        if ((mode&MODE_WORLD_WRITEABLE) != 0) {
            perms |= FileUtils.S_IWOTH;
        }
        if (DEBUG) {
            Log.i(TAG, "File " + name + ": mode=0x" + Integer.toHexString(mode)
                  + ", perms=0x" + Integer.toHexString(perms));
        }
        FileUtils.setPermissions(name, perms, -1, -1);
    }

　　从上面代码可以知道为何新建的数据库的目录为/data/data/<PackageName>/database/，并且权限为-rx-rx--，再贴下之前新建数据库得到的结果：

　　其中SQLiteDatabase db = SQLiteDatabase.openDatabase(f.getPath(), factory, flags, errorHandler);应该是正真建立数据库的地方。

    public static SQLiteDatabase openDatabase(String path, CursorFactory factory, int flags,
            DatabaseErrorHandler errorHandler) {
        SQLiteDatabase db = new SQLiteDatabase(path, flags, factory, errorHandler);
        db.open();
        return db;
    }

　　这样最终就可以得到SQLite数据库了。