Android getReadableDatabase() 和 getWritableDatabase()分析对比
时间:2019-03-30
本文章向大家介绍Android getReadableDatabase() 和 getWritableDatabase()分析对比,主要包括Android getReadableDatabase() 和 getWritableDatabase()分析对比使用实例、应用技巧、基本知识点总结和需要注意事项,具有一定的参考价值,需要的朋友可以参考一下。
Android getReadableDatabase() 和 getWritableDatabase()分析对比
Android使用getWritableDatabase()和getReadableDatabase()方法都可以获取一个用于操作数据库的SQLiteDatabase实例。(getReadableDatabase()方法中会调用getWritableDatabase()方法)
其中getWritableDatabase() 方法以读写方式打开数据库,一旦数据库的磁盘空间满了,数据库就只能读而不能写,倘若使用的是getWritableDatabase() 方法就会出错。
getReadableDatabase()方法则是先以读写方式打开数据库,如果数据库的磁盘空间满了,就会打开失败,当打开失败后会继续尝试以只读方式打开数据库。如果该问题成功解决,则只读数据库对象就会关闭,然后返回一个可读写的数据库对象。
源码如下:
/** * 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; if (mDatabase != null) mDatabase.lock(); try { mIsInitializing = true; if (mName == null) { db = SQLiteDatabase.create(null); } else { db = mContext.openOrCreateDatabase(mName, 0, 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.unlock(); } mDatabase = db; } else { if (mDatabase != null) mDatabase.unlock(); 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 = mContext.getDatabasePath(mName).getPath(); db = SQLiteDatabase.openDatabase(path, mFactory, SQLiteDatabase.OPEN_READONLY); 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(); } }
感谢阅读,希望能帮助到大家,谢谢大家对本站的支持!
- CTF学习交流群 第一期入群题writeup大放送
- 如何使用Hue创建Spark1和Spark2的Oozie工作流
- 【译】深入研究 Laravel 的依赖注入容器
- 一次XSS突破的探险
- 如何使用Hue创建Spark2的Oozie工作流(补充)
- 如何基于CDSW基础镜像定制Docker
- OVSDB介绍及在OpenDaylight中的调用
- 如何在CDH集群的非元数据库节点安装MySQL5.7.12
- PySpark数据类型转换异常分析
- SQLI-LABS 更新帖(二)
- 如何重置Hue用户密码
- 如何使用R连接Hive与Impala
- Equation Group泄露工具之vBulletin无文件后门分析
- 如何使用CDSW在CDH集群通过sparklyr提交R的Spark作业
- java教程
- Java快速入门
- Java 开发环境配置
- Java基本语法
- Java 对象和类
- Java 基本数据类型
- Java 变量类型
- Java 修饰符
- Java 运算符
- Java 循环结构
- Java 分支结构
- Java Number类
- Java Character类
- Java String类
- Java StringBuffer和StringBuilder类
- Java 数组
- Java 日期时间
- Java 正则表达式
- Java 方法
- Java 流(Stream)、文件(File)和IO
- Java 异常处理
- Java 继承
- Java 重写(Override)与重载(Overload)
- Java 多态
- Java 抽象类
- Java 封装
- Java 接口
- Java 包(package)
- Java 数据结构
- Java 集合框架
- Java 泛型
- Java 序列化
- Java 网络编程
- Java 发送邮件
- Java 多线程编程
- Java Applet基础
- Java 文档注释
- 【腾讯云】记一次搭建云服务器
- 前端:如何让background背景图片进行CSS自适应
- 为什么Rust的println!不会发生所有权转移?
- 【译文】Rust futures: async fn中的thread::sleep和阻塞调用
- Smart Pointer Programming Techniques
- 一文搞懂AQS及其组件的核心原理
- 比较JavaScript中的数据结构(数组与对象)
- 9 个JavaScript 技巧
- [已解决]报错:XGBoostError: XGBoost Library (libxgboost.dylib) could not be loaded.
- 2.3 spring5源码---spring ioc 加载配置类的源码
- Tree Shaking概念详解
- Canvas基础教程(章节1)
- Canvas基础教程(章节2)
- CSS文件夹
- Canvas基础教程(章节3)