兩個小時精通Android開發之數據持久存儲篇
作者：孫東風 2010-01-15（轉載務必註明出處）
筆者在前面的兩篇文章《兩個小時精通Android開發之界面篇》、《兩個小時精通Android開發之按鍵映射篇》分別講了無縫移植J2ME程序到Android平臺上對界面和用戶按鍵交互所做的適配接口，原則上利用這些接口原有的J2ME程序基本不用做任何的修改就可以運行在Android平臺上，所以精通J2ME也就等於精通了Android。
筆者這篇文章裏要講述的是J2ME平臺和Android平臺另外一個重要的不同點，那就是數據持久存儲系統。
J2ME平臺裏採用RMS系統進行數據的持久存儲，而Android平臺則提供了豐富的接口進行數據的持久存儲，但任何持久存儲的本質無非就是數據串行化後被保存到磁盤空間上，仔細研究J2ME平臺RMS系統的實現源碼可以看到，J2ME是通過一個叫做RecordStoreFile的類進行數據持久化存儲的，而這個RecordStoreFile類的實現如下：
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.addElement(File.asciiFilenameToUnicode(asciiName));
}
}
}
if (names.size() == 0) {
return null;
}
String[] rv = new String[names.size()];
names.copyInto(rv);
return rv;
}
public static void removeRecordStoresForSuite(SecurityToken token,
String suiteStorageRoot) {
File storage;
String[] filenames;
storage = new File(token);
filenames = listRecordStoresForSuite(storage, suiteStorageRoot, true);
if (filenames == null) {
return;
}
for (int i = 0; i < filenames.length; i++) {
try {
storage.delete(filenames[i]);
} catch (IOException ioe) {
// move on to the next suite
}
}
}
public static boolean suiteHasRmsData(String suiteStorageRoot) {
File storage = new File(classSecurityToken);
Vector files = storage.filenamesThatStartWith(suiteStorageRoot);
for (int i = 0; i < files.size(); i++) {
String file = (String)files.elementAt(i);
if (file.endsWith(dbExtension)) {
return true;
}
}
return false;
}
public static int spaceAvailable()
{
return new File(classSecurityToken).getBytesAvailableForFiles();
}
private static String getStoragePath(String name)
{
String str;
MIDletSuite mSuite;
StringBuffer path;
mSuite = Scheduler.getScheduler().getMIDletSuite();
str = mSuite.getStorageRoot();
if (name != null) {
path = new StringBuffer(str);
// convert the unicode filename into a system acceptable string
path.append(File.unicodeToAsciiFilename(name));
path.append(dbExtension);
str = path.toString();
}
return str;
}
private static String getStoragePath(String vendor, String suite,
String name) {
String str = File.getStorageRoot();
StringBuffer path = new StringBuffer(str);
if (vendor != null && suite != null) {
path.append(File.unicodeToAsciiFilename(vendor));
path.append('_');
path.append(File.unicodeToAsciiFilename(suite));
path.append('_');
}
if (name != null) {
path.append(File.unicodeToAsciiFilename(name));
path.append(dbExtension);
str = path.toString();
}
return str;
}
}
可見，RMS系統也是通過IO把數據串行化後存儲應用程序的空間內的，絕大多數的J2ME程序都需要利用RMS來進行數據的持久存儲的，比如遊戲積分、系統設置等。那麼爲了無縫移植J2ME到Android平臺，筆者自己寫了一個簡易的RMS系統，能滿足絕大多數應用程序的需要。
Android下對文件的操作和J2ME基本一樣，但是需要綁定一個Context上下文，以把文件保存到當前應用程序的目錄下，這個目錄在打開DDMS窗口後可以看到，具體位置是data/data/PACKAGE_NAME/files下面。
利用文件操作可以進行數據的讀寫，筆者封裝的接口如下：
public static boolean deleteFile(String fileName){
File file = new File(fileName);
if(file.isFile() && file.exists()){
file.delete();
System.out.println("刪除單個文件"+fileName+"成功！");
return true;
}else{
System.out.println("刪除單個文件"+fileName+"失敗！");
return false;
}
}
public String read(String file) {
String data = "";
try {
FileInputStream stream = System.getSystemHandler().getContext().openFileInput(file);
StringBuffer sb = new StringBuffer();
int c;
while ((c = stream.read()) != -1) {
sb.append((char) c);
}
stream.close();
data = sb.toString();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return data;
}
public void write(String file, byte[] msg) {
try {
FileOutputStream stream = System.getSystemHandler().getContext().openFileOutput(file,Context.MODE_WORLD_WRITEABLE);
stream.write(msg);
stream.flush();
stream.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
有了基本的讀寫數據操作，就可以封裝RMS中常見的key-value的保存和讀取了，代碼實現如下：
public boolean put(String key, byte[] value) {
boolean bSaveOk = false;
byte[] data = null;
if (value == null) {
throw new NullPointerException();
}
ByteArrayOutputStream bout = null;
DataOutputStream dout = null;
try {
bout = new ByteArrayOutputStream();
dout = new DataOutputStream(bout);
dout.writeUTF(key);
dout.writeInt(value.length);
dout.write(value, 0, value.length);
data = bout.toByteArray();
write(dbName,data);
bSaveOk = true;
} catch (Exception e) {
bSaveOk = false;
e.printStackTrace();
}
return bSaveOk;
}
public byte[] getByteArray(String key) {
ByteArrayInputStream bin = null;
DataInputStream din = null;
byte[] data = null;
try {
String valueKey = read(dbName);
din = new DataInputStream(new ByteArrayInputStream(valueKey.getBytes()));
while(din.available() > 0)
{
String getKey = din.readUTF();
int getLength = din.readInt();
data = new byte[getLength];
int bytesRead = 0;
while (bytesRead < data.length) {
int count = din.read(data, bytesRead, data.length
- bytesRead);
if (count == -1)
break;
bytesRead += count;
}
if(getKey.equals(key))
break;
}
din.close();
din = null;
} catch (Exception e) {
e.printStackTrace();
data = null;
}
return data;
}
}