JavaMe 编程连载(7) - 数据永久存储

【问题描述】JavaMe不支持关系型数据库的操作，如何实现永久存储呢？

JavaMe中，之所以没有不支持数据库，是因为功能性手机一般存储空间较小。所以采用一种成为“记录”的概念代替，即采用一种成为RMS的机制进行永久存储。

 

【实例】

UserDataItem.java

package com.token.model;

import com.token.util.StringDealMethod;

public class UserDataItem {
	private int id;
    public String name = null;
    public String passwd = null;

    public UserDataItem(String name,String passwd)
    {
    	this.name = name;
    	this.passwd = passwd;	
    }
    
    public UserDataItem(int id,byte[] data){
        this.id=id;
    	String temp=new String(data);
    	String temp_sub[] = StringDealMethod.split(temp, ",");
    	this.name = temp_sub[0];
    	this.passwd = temp_sub[1];
    }
    
    public int getId(){
    	return id;
    }
    
    public void setId(int id){
    	this.id=id;
    }
    
    public String getName(){
        return name;
    }
    
    public void setName(String name){
        this.name = name;
    }
    
    public String getPasswd(){
        return passwd;
    }
    
    public void setPasswd(String passwd){
        this.passwd = passwd;
    }
    
    public byte[] getBytes(){
        String temp=null;
        
        if(name==null||passwd==null){
            return null;
        }else{
            temp=name+","+passwd;
        }
        
        return temp.getBytes();
    }
}

UserDataRecord.java

package com.token.util;

import java.util.Vector;

import javax.microedition.rms.RecordComparator;
import javax.microedition.rms.RecordEnumeration;
import javax.microedition.rms.RecordStore;
import javax.microedition.rms.RecordStoreException;

import com.token.model.*;

public class UserDataRecord {
	private static final String RECORDSTORE_NAME="USER_DB";
	private static RecordStore info;
	
	public UserDataRecord(){
	}
	
	//打开RecordStore，没有则创建
    public void openDataBase() {
        try {
            info = RecordStore.openRecordStore(RECORDSTORE_NAME, true);
        }catch (RecordStoreException ex) {            
            info =null;
        }
    }
    //关闭RecordStore
    public void closeDataBase() {
        if (info!= null) {
            try {
                info.closeRecordStore();
                info=null;
            } catch (RecordStoreException ex) {}
        }
    }
    //增加记录
    public int db_addRecord(UserDataItem item) {
                   
                try {
                    this.openDataBase();
                    byte[] data=item.getBytes();
                    int id=info.getNextRecordID();
                	info.addRecord(data,0,data.length);
                	this.closeDataBase();
                	return id;
                } catch (RecordStoreException ex) { }
        
        return -1;
    }
    //更新记录
    public void db_updateRecord(UserDataItem item){

            try {
            	this.openDataBase();      
                byte[] data=item.getBytes();
            	info.setRecord(item.getId(),data,0,data.length);
            	this.closeDataBase();
            } catch (RecordStoreException ex) { }
    }
    //访问一条记录
    public UserDataItem db_getRecord(int id){
    	UserDataItem item=null;

            try {
                this.openDataBase();
            	item = new UserDataItem(id,info.getRecord(id));
            	this.closeDataBase();
            } catch (RecordStoreException ex) { }

         return item;
    }
    //删除一条记录
    public void db_deleteRecord(int id){
            try {
                this.openDataBase();
            	info.deleteRecord(id);
            	this.closeDataBase();
            } catch (RecordStoreException ex) {}
    }
    
    //删除所有记录
    public void db_deleteAllRecord(){
            try {
            	RecordStore.deleteRecordStore(RECORDSTORE_NAME);
            } catch (RecordStoreException ex) {}
    }
    
    //访问所有记录
    public Vector db_getRecords(){
        Vector items=new Vector(10,3);
        this.openDataBase();//打开RecordStore
        RecordEnumeration enum1=null;
        int ind=0;
        try{
            UserDataItem item=null;
        	enum1=info.enumerateRecords(null,new InnerComparator(),false);
            
            while(enum1.hasPreviousElement()){
            	ind=enum1.previousRecordId();
        	    item=new UserDataItem(ind,info.getRecord(ind));
            	items.addElement(item);
            }
            
        }catch(Exception ex){ex.printStackTrace();}
        finally{
        		try{
        			enum1.destroy();
        		}catch(Exception e){}
        		
        		this.closeDataBase();//关闭RecordStore
        }//end finally
    
        return items;
    }
    //一个简单的比较器
    private class InnerComparator implements RecordComparator{
        public int compare(byte[] rec1, byte[] rec2){
            if(rec1.length>rec2.length)
                return FOLLOWS;
            else if(rec1.length<rec2.length)
                return PRECEDES;
            else 
                return EQUIVALENT;
        }

    }
}

【分析】

我们模拟了一种数据库，下以“数据库”称之，实际上还是对记录进行操作。

1 打开数据库

public void openDataBase() {
        try {
            info = RecordStore.openRecordStore(RECORDSTORE_NAME, true);
        }catch (RecordStoreException ex) {            
            info =null;
        }
    }

2 关闭数据库

public void closeDataBase() {
        if (info!= null) {
            try {
                info.closeRecordStore();
                info=null;
            } catch (RecordStoreException ex) {}
        }
    }

3 增加记录(增)

public int db_addRecord(UserDataItem item) {
                   
                try {
                    this.openDataBase();
                    byte[] data=item.getBytes();
                    int id=info.getNextRecordID();
                	info.addRecord(data,0,data.length);
                	this.closeDataBase();
                	return id;
                } catch (RecordStoreException ex) { }
        
        return -1;
    }

4 更新记录(改)

 //更新记录
    public void db_updateRecord(UserDataItem item){

            try {
            	this.openDataBase();      
                byte[] data=item.getBytes();
            	info.setRecord(item.getId(),data,0,data.length);
            	this.closeDataBase();
            } catch (RecordStoreException ex) { }
    }

5 查看记录(查)

//访问一条记录
    public UserDataItem db_getRecord(int id){
    	UserDataItem item=null;

            try {
                this.openDataBase();
            	item = new UserDataItem(id,info.getRecord(id));
            	this.closeDataBase();
            } catch (RecordStoreException ex) { }

         return item;
    }

6 删除记录

 public void db_deleteRecord(int id){
            try {
                this.openDataBase();
            	info.deleteRecord(id);
            	this.closeDataBase();
            } catch (RecordStoreException ex) {}
    }

7 删除所有记录

public void db_deleteAllRecord(){
            try {
            	RecordStore.deleteRecordStore(RECORDSTORE_NAME);
            } catch (RecordStoreException ex) {}
    }
    

8 如何使用？

(1) 新建对象

private UserDataRecord userRecord;

userRecord = new UserDataRecord();

(2) 添加记录

UserDataItem userItem = new UserDataItem(1,(username+","+passwd).getBytes());
userRecord.db_addRecord(userItem);

(3) 查看记录

userRecord.db_getRecord(1);

*1为记录id，在添加记录步骤中，id为1。

 

(4) 更新记录

UserDataItem userItem = new UserDataItem(1,(username+","+passwd).getBytes());
userRecord.db_updateRecord(userItem);

 

(5) 综合实例

userRecord.db_deleteAllRecord();
if(userRecord.db_getRecord(1)==null)
{
	UserDataItem userItem = new UserDataItem(1,(username+","+passwd).getBytes());
	userRecord.db_addRecord(userItem);
	userItem = null;
}

 

*在添加记录前，要检测当前记录ID是否被使用过，否则“增”操作会失败。如果不指定数值，默认情况下，id会递增1。所以在多次调用时，而又只需要一条记录的情况下，调用了userRecord.db_deleteAllRecord();

*将UserDataRecord.java中的UserDataItem改为Object对象，稍作修改，可作为一个通用的数据库，有兴趣的朋友可以试一试。

 

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