两个小时精通Android开发之数据持久存储篇


		作者：孙东风 2010-01-15（转载务必注明出处）
 
笔者在前面的两篇文章《两个小时精通Android开发之界面篇》、《两个小时精通Android开发之按键映射篇》分别讲了无缝移植J2ME程序到Android平台上对界面和用户按键交互所做的适配接口，原则上利用这些接口原有的J2ME程序基本不用做任何的修改就可以运行在Android平台上，所以精通J2ME也就等于精通了Android。
 
笔者这篇文章里要讲述的是J2ME平台和Android平台另外一个重要的不同点，那就是数据持久存储系统。
 
J2ME平台里采用RMS系统进行数据的持久存储，而Android平台则提供了丰富的接口进行数据的持久存储，但任何持久存储的本质无非就是数据串行化后被保存到磁盘空间上，仔细研究J2ME平台RMS系统的实现源码可以看到，J2ME是通过一个叫做RecordStoreFile的类进行数据持久化存储的，而这个RecordStoreFile类的实现如下：
 
public class RecordStoreFile {
   private static SecurityToken classSecurityToken;
    private static final String dbExtension = ".db";
    private RandomAccessStream recordStream;
    private String myStoragePath;
 
    public static void initSecurityToken(SecurityToken token) {
    if (classSecurityToken != null) {
        return;
    }
    classSecurityToken = token;
    }
 
    public RecordStoreFile(String uidPath) 
    throws IOException 
    {
    RandomAccessStream newStream;
    myStoragePath = uidPath;
 
    newStream = new RandomAccessStream(classSecurityToken);
    newStream.connect(myStoragePath, Connector.READ_WRITE);
    recordStream = newStream;
    }
 
    public static String getUniqueIdPath(String fileName) {
    return getStoragePath(fileName);
    }
    
    public static String getUniqueIdPath(String vendorName, String suiteName, 
                     String fileName) {
    return getStoragePath(vendorName, suiteName, fileName);
    }
 
    public static boolean exists(String uidPath) {
    File file;
    file = new File(classSecurityToken);
    return file.exists(uidPath);
    }
 
    
    public static boolean deleteFile(String uidPath)
    {
    File file;
    file = new File(classSecurityToken);
    try {
        file.delete(uidPath);
        return true;
    } catch (IOException ioe) {
        return false;
    }
    }
    
    public void seek(int pos) throws IOException
    {
    recordStream.setPosition(pos);
    }
 
    public void write(byte[] buf) throws IOException
    {
    write(buf, 0, buf.length);
    }
  
    public void write(byte[] buf, int offset, int numBytes) throws IOException
    {
    recordStream.writeBytes(buf, offset, numBytes);
    }
    
    public int read(byte[] buf) throws IOException
    {
    return read(buf, 0, buf.length);
    }
 
    public int read(byte[] buf, int offset, int numBytes) throws IOException
    {
    return recordStream.readBytes(buf, offset, numBytes);
    }
  
    public void close() throws IOException
    {
    // close recordStream if it exists
    if (recordStream != null) {
        recordStream.disconnect();
        recordStream = null;
    }
    }
 
    public void truncate(int size) throws IOException
    {
    if (recordStream != null) {    
        recordStream.truncate(size);
    }    
    }
 
    public static String[] listRecordStores() {
        return listRecordStoresForSuite(new File(classSecurityToken),
            getStoragePath(null), false);
    }
 
    private static String[] listRecordStoresForSuite(File storage,
                                                     String suiteStorageRoot,
                                                     boolean rawNames) {
    Vector files;
    Vector names;
    String file;
    String asciiName;
    
    files = storage.filenamesThatStartWith(suiteStorageRoot);
    names = new Vector();
    
    // work through list of strings from the directory
    for (int i = 0; i < files.size(); i++) {
        file = (String)files.elementAt(i);
        if (file.endsWith(dbExtension)) {
                if (rawNames) {
                    names.addElement(file);
                } else {
                    asciiName = file.substring(suiteStorageRoot.length(), 
                       file.length() - 3);
                    names.add

补充：移动开发 , Android ,