javaSe_ID的生成模式

思路：取出一个主键value，让其value++,再存入DB中
 下面这个代码注意3点：1.对于Connection 这类打开资源的最好采用传递的方式；2对于自己private 私用的类的异常最好是上抛;3在手动throw new RuntimeException("生成Id出错！")这样好让程序不再往入执行；4最小化的加锁不能同步代码太多,synchronized long newID(String tableName)加锁synchronized long newID(String tableName)，静方法加锁是属于类，成员方法加锁是属于对象,这样保证了同一时间只有一个人在用;5下面这个是采用了DB中的悲观锁来做的，这样就不能让rs = pstmt.executeQuery();自动来commit事务,这要设置为自己提交事务;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import com.bjpowernode.drp.util.DBUtil;

/**
* id生成器,采用单例模式
* @author Administrator
*
*/
public class IDGenerator {

private static IDGenerator instance = new IDGenerator();

private IDGenerator() {}

public static IDGenerator getInstance() {
return instance;
}

/**
* 生成新id
* @param tableName
* @return
*/
//public synchronized long newID(String tableName) {
//synchronized(this) {
public long newID(String tableName) {
String sql = "select value from t_table_id where table_name=? for update";
Connection conn = null;
PreparedStatement pstmt = null;
ResultSet rs = null;
long value = 0L;
try {
conn = DBUtil.getConnection();
//开启事务
DBUtil.setAutoCommit(conn, false);
pstmt = conn.prepareStatement(sql);
pstmt.setString(1, tableName);
rs = pstmt.executeQuery();
if (rs.next()) {
value = rs.getLong("value");
}else {
throw new RuntimeException("生成Id出错！");
}
value = value + 1;

//更新value
modifyValueField(conn, tableName, value);
//提交事务
DBUtil.commit(conn);
}catch(Exception e) {
e.printStackTrace();
//回滚事务
DBUtil.rollback(conn);
throw new RuntimeException("生成Id出错！");
}finally {
DBUtil.close(rs);
DBUtil.close(pstmt);
DBUtil.close(conn);
}
return value;
}

/**
* 更新value
* @param conn
* @param tableName
* @param value
* @throws SQLException
*/
private void modifyValueField(Connection conn, String tableName, long value) 
throws SQLException {
String sql = "update t_table_id set value=? where table_name=?";
PreparedStatement pstmt = null;
try {
pstmt = conn.prepareStatement(sql);
pstmt.setLong(1, value);
pstmt.setString(2, tableName);
pstmt.executeUpdate();
}finally {
DBUtil.close(pstmt);
}
} 

public static void main(String[] args) {
long newId = IDGenerator.getInstance().newID("t_client");
System.out.println(newId);
}
}