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J2ME record management store
The Mobile Information Device Profile -- the platform for mobile Java applications -- provides a mechanism for MIDP applications to persistently store data across multiple invocations. This persistent storage mechanism can be viewed as a simple record-oriented database model and is called the record management system (RMS). Here, Manish Pathak shows how your J2ME application can use RMS to manage and interpret data. You'll also get a demonstration of the concept through a sample phone database.
The J2ME record management system (RMS) provides a mechanism through which MIDlets can persistently store data and retrieve it later. In a record-oriented approach, J2ME RMS comprises multiple record stores. An overview of J2ME RMS and MIDlet interfacing is given in Figure 1.
Figure 1. Overview of J2ME RMS and MIDlet interfacing
Each record store can be visualized as a collection of records, which will remain persistent across multiple invocations of the MIDlet. The device platform is responsible for making its best effort to maintain the integrity of the MIDlet's record stores throughout the normal use of the platform, including reboots, battery changes, etc.
A record store is created in platform-dependent locations, like nonvolatile device memory, which are not directly exposed to the MIDlets. The RMS classes call into the platform-specific native code that uses the standard OS data manager functions to perform the actual database operations.
Record store implementations ensure that all individual record store operations are atomic, synchronous, and serialized, so no corruption of data will occur with multiple accesses. The record store is timestamped to denote the last time it was modified. The record store also maintains a version, which is an integer that is incremented for each operation that modifies the contents of the record store. Versions and timestamps are useful for synchronization purposes.
When a MIDlet uses multiple threads to access a record store, it is the MIDlet's responsibility to coordinate this access; if it fails to do so, unintended consequences may result. Similarly, if a platform performs a synchronization of a record store with multiple threads trying to access the record store simultaneously, it is the platform's responsibility to enforce exclusive access to the record store between the MIDlet and its synchronization engine.
Each record in a record store is an array of bytes and has a unique integer identifier.
The
javax.microedition.rms.RecordStore class represents a RMS record store. It provides several methods to manage as well as insert, update, and delete records in a record store.To open a record store, the
openRecordStore() method of javax.microedition.rms.RecordStore is invoked. public static RecordStore openRecordStore(String recordStoreName, boolean createIfNecessary) opens a record store with the given name recordStoreName. If there is no record store of that name, invoking this method creates one.If the record store is already open, this method returns a reference to the same record store object.
Listing 1. Open a RecordStore
RecordStore rs = RecordStore.openRecordStore("MyAppointments",true); |
Once all operations are done, a call to
closeRecordStore() closes the record store with the given name. When a record store is closed, it is disabled for further operations.Listing 2. Close a RecordStore
Rs.closeRecordStore(); |
A named record store can be deleted by invoking the
deleteRecordStore() method.Listing 3. Delete a RecordStore
RecordStore.deleteRecordStore("MyAppointments"); |
Inserting records
The MIDlet invokes the
addRecord() method of javax.microedition.rms.RecordStore class to insert a new record into the record store. This is a blocking atomic operation and returns the recordId for the new record. The record is written to persistent storage before the method returns.public int addRecord(byte[] data, int offset, int numBytes) inserts a record represented by an array of bytes data with offset as its starting index and numBytes as its length.Listing 4. Insert a record
String appt = "new record";
byte bytes[] = appt.getBytes();
rs.addRecord(bytes,0,bytes.length); |
Updating records
Updating a particular record involves getting a handle for that record and setting new information.
public int getRecord(int recordId, byte[] buffer, int offset) returns the data stored in the given record in the byte array represented by buffer. public byte[] getRecord(int recorded) returns a copy of the data represented by recordId. public void setRecord(int recordId, byte[] newData, int offset, int numBytes) sets new information, a stream of bytes (newData) with offset as its starting index and numBytes as its length, at the record location represented by recordId.Listing 5. Update a record
String newappt = "update record"; Byte data = newappt.getBytes(); Rs.setRecord(1, data, 0, data.length()); |
Deleting records
The MIDlet invokes the
deleteRecord() method to delete a record from the record store.public void deleteRecord(int recordId) deletes the record represented by recordId. The recordId for this record is not subsequently reused.Listing 6. Delete a record
Rs.deleteRecord(1); |
The J2ME API provides certain interfaces to interpret the data stored in a record store. This process involves comparing records to determine their relative sort order. It also involves the filtering of contents depending on given conditions.
Comparing records
The MIDlet implements the
RecordComparator interface and defines a compare (byte[] rec1, byte[] rec2) method to compare two candidate records. The return value of this method must indicate the ordering of the two records.Listing 7. Comparing records and determine relative sort order
Int compare (byte[] b1, byte[] b2)
{
String s1 = new String(b1);
String s2 = new String(b2);
If (s1.compareTo(s2) > 0)
Return RecordComparator.FOLLOWS;
Else if (s1.compareTo(s2) == 0)
Return RecordComparator.EQUIVALENT;
Else
Return RecordComparator.PRECEDES;
} |
Enumerating records
The
RecordEnumeration interface is responsible for enumerating records in a record store. It logically maintains a sequence of the recordIds of the records in a record store. The enumerator will iterate over all of the records (or only a subset, if an optional record filter has been supplied) in an order determined by a record comparator. If neither the filter nor the comparator are specified, the enumeration will traverse all records in the record store in an undefined order.Listing 8. Enumerating records
RecordEnumeration re = rs.enumerateRecords(null, null, false);
If (re.hasNextElement())
Byte nextRec[] = re.nextRecord(); |
Filtering records
The MIDlet implements the
RecordFilter interface, defining a filter that examines a record to see if it meets application-defined criteria. The application implements the RecordFilter's match() method to select the records to be returned by the RecordEnumeration.Listing 9. Filtering records
Public boolean matches(byte[] candidate)
{
String s1 = new String(candidate);
If (s1.equals("XX"))
Returns true;
Else
Returns false;
}In below code,
1- open the connection
2- perform the operation(store or retrieve)
3- close the connection
For reading a record from record store ,call method as-
StorageUtility.readRecord("my_record_store",false);
For writing a record from record store ,call method as-
StorageUtility.readRecord("my_record_store","my_record",true);
SAMPLE CODE : StorageUtility.java
public class StorageUtility
{
static RecordStore tjRecordStore=null;
static int recordId = 1;
private static RecordStore getRecordStore(String recordStoreName,boolean createIFNecessary )
{
try
{
tjRecordStore = RecordStore.openRecordStore(recordStoreName, createIFNecessary);
}
catch (Exception e)
{
tjRecordStore=null;
e.printStackTrace();
}
return tjRecordStore;
}
private static void closeStore(RecordStore recStore)
{
try
{
if(recStore != null)
{
recStore.closeRecordStore();
}
}
catch (Exception e)
{
e.printStackTrace();
}
}
private static int addRecord(RecordStore recStore,String record)
{
byte[] data = record.getBytes();
try
{
recordId = recStore.addRecord(data, 0, data.length);
}
catch (Exception e)
{
e.printStackTrace();
}
return recordId;
}
private static void updateRecord(RecordStore recStore,String record,int recordId)
{
byte[] data = record.getBytes();
try
{
recStore.setRecord(recordId, data, 0, data.length);
}
catch (Exception e)
{
e.printStackTrace();
}
}
private static String getRecord(RecordStore recStore,int recordId)
{
String recStr="";
try
{
byte[] data = recStore.getRecord(recordId);
recStr = new String(data,0,data.length);
}
catch (Exception e)
{
e.printStackTrace();
}
return recStr;
}
private static void deleteRecord(RecordStore recStore,int recordId)
{
try
{
recStore.deleteRecord(recordId);
}
catch (Exception e)
{
e.printStackTrace();
}
}
private static void deleteRecordStore(String recordStoreName)
{
try
{
RecordStore.deleteRecordStore(recordStoreName);
}
catch (Exception e)
{
e.printStackTrace();
}
}
public static void writeRecord(String recordStoreName,String record, boolean createIFNecessary)
{
tjRecordStore = getRecordStore(recordStoreName,createIFNecessary);
addRecord(tjRecordStore,record);
closeStore(tjRecordStore);
}
public static String readRecord(String recordStoreName,boolean createIFNecessary)
{
tjRecordStore = getRecordStore(recordStoreName,createIFNecessary);
String record = getRecord(tjRecordStore,recordId);
closeStore(tjRecordStore);
return record;
}
public static int getNumberOfRecords(RecordStore recStore)
{
int records = 0;
try
{
records = recStore.getNumRecords();
}
catch (Exception e)
{
e.printStackTrace();
}
return records;
}
}
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