题目是ConnectionManager管理Connection对象的固定数组,客户端不能直接创建Connection对象,只能通过ConnectionManager中的某个static方法来获取,当ConnectionManager中不再有对象时,返回null引用。
//mypackage.Connection.java
package mypackage;
public class Connection {
private static int i=0;
public Connection() {
System.out.println("i="+(++i));
}
public static int getNum() {
return i;
}
public static void clearNum() {
if(i!=0) i--;
else System.out.println("Please set up a Connection first!");
}
public String toString() {
return "set up Con,i="+i;
}
}//(默认包)/ConnectionManager.java
import mypackage.Connection;
public class ConnectionManager {
Connection[] num=new Connection[6];
Connection getCon() {
System.out.println(Connection.getNum());
if(Connection.getNum()!=0) {
Connection.clearNum();
return num[Connection.getNum()-1];
}
else return null;
}
public static void main(String[] args) {
ConnectionManager con=new ConnectionManager();
for(int i=0;i<10;i++) {
System.out.println(con.getCon());
}
}
}
上面是我写的程序,因为初学所以搞得很混乱。。程序可以运行可是得不到想要的结果,最让我想得头大的是Connection的构造函数根本就没有调用……希望高手帮忙解答一下呀。。谢谢```````
//mypackage.Connection.java
package mypackage;
public class Connection {
private static int i=0;
public Connection() {
System.out.println("i="+(++i));
}
public static int getNum() {
return i;
}
public static void clearNum() {
if(i!=0) i--;
else System.out.println("Please set up a Connection first!");
}
public String toString() {
return "set up Con,i="+i;
}
}//(默认包)/ConnectionManager.java
import mypackage.Connection;
public class ConnectionManager {
Connection[] num=new Connection[6];
Connection getCon() {
System.out.println(Connection.getNum());
if(Connection.getNum()!=0) {
Connection.clearNum();
return num[Connection.getNum()-1];
}
else return null;
}
public static void main(String[] args) {
ConnectionManager con=new ConnectionManager();
for(int i=0;i<10;i++) {
System.out.println(con.getCon());
}
}
}
上面是我写的程序,因为初学所以搞得很混乱。。程序可以运行可是得不到想要的结果,最让我想得头大的是Connection的构造函数根本就没有调用……希望高手帮忙解答一下呀。。谢谢```````
import java.sql.*;
import java.util.*;
import java.util.Date;/**
* 管理类DBConnectionManager支持对一个或多个由属性文件定义的数据库连接
* 池的访问.客户程序可以调用getInstance()方法访问本类的唯一实例.
*/
public class ConnectPool
{
static public ConnectPool instance; // 唯一实例
static public int clients;
public Vector drivers = new Vector(); // 驱动
public PrintWriter log;
public Hashtable pools = new Hashtable(); // 连接/**
* 返回唯一实例.如果是第一次调用此方法,则创建实例
*
* @return DBConnectionManager 唯一实例
*/
static synchronized public ConnectPool getInstance()
{
if (instance == null)
{
instance = new ConnectPool();
}
clients++;
return instance;
}/**
* 建构函数私有以防止其它对象创建本类实例
*/
public ConnectPool() {
init();
}/**
* 将连接对象返回给由名字指定的连接池
*
* @param name
* 在属性文件中定义的连接池名字
* @param con
* 连接对象
*/
public void freeConnection(String name, Connection con)
{
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null)
{
pool.freeConnection(con);
}
else
{
System.out.println("pool ==null");
}
clients--;
}/**
* 获得一个可用的(空闲的)连接.如果没有可用连接,且已有连接数小于最大连接数 限制,则创建并返回新连接
*
* @param name
* 在属性文件中定义的连接池名字
* @return Connection 可用连接或null
*/
public Connection getConnection(String name)
{
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null)
{
// return pool.getConnection();
return pool.returnConnection();
}
return null;
}/**
* 获得一个可用连接.若没有可用连接,且已有连接数小于最大连接数限制, 则创建并返回新连接.否则,在指定的时间内等待其它线程释放连接.
*
* @param name
* 连接池名字
* @param time
* 以毫秒计的等待时间
* @return Connection 可用连接或null
*/
public Connection getConnection(String name, long time)
{
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null)
{
return pool.getConnection(time);
}
return null;
}/**
* 关闭所有连接,撤销驱动程序的注册
*/
public synchronized void release()
{
// 等待直到最后一个客户程序调用
if (--clients != 0)
{
return;
}
Enumeration allPools = pools.elements();
while (allPools.hasMoreElements())
{
DBConnectionPool pool = (DBConnectionPool) allPools.nextElement();
pool.release();
}
Enumeration allDrivers = drivers.elements();
while (allDrivers.hasMoreElements())
{
Driver driver = (Driver) allDrivers.nextElement();
try {
DriverManager.deregisterDriver(driver);
log("撤销JDBC驱动程序 " + driver.getClass().getName()+"的注册");
}
catch (SQLException e)
{
log(e, "无法撤销下列JDBC驱动程序的注册: " + driver.getClass().getName());
}
}
}/**
* 根据指定属性创建连接池实例.
*
* @param props
* 连接池属性
*/
private void createPools(Properties props)
{
Enumeration propNames = props.propertyNames();
while (propNames.hasMoreElements())
{
String name = (String) propNames.nextElement();
if (name.endsWith(".url")) {
String poolName = name.substring(0, name.lastIndexOf("."));
String url = props.getProperty(poolName + ".url");
if (url == null) {
log("没有为连接池" + poolName + "指定URL");
continue;
}
String user = props.getProperty(poolName + ".user");
String password = props.getProperty(poolName + ".password");
String maxconn = props.getProperty(poolName + ".maxconn", "0");
int max;
try{
max = Integer.valueOf(maxconn).intValue();
}
catch (NumberFormatException e)
{
log("错误的最大连接数限制: " + maxconn + " .连接池: " + poolName);
max = 0;
}
DBConnectionPool pool = new DBConnectionPool(poolName, url, user, password, max);
pools.put(poolName, pool);
log("成功创建连接池" + poolName);
}
}
}/**
* 读取属性完成初始化
*/
private void init()
{
try
{
Properties p = new Properties();
String configs = System.getProperty("user.dir")+"confdb.properties";
System.out.println("configs file local at "+configs);
FileInputStream is = new FileInputStream(configs);
Properties dbProps = new Properties();
try
{
dbProps.load(is);
}
catch (Exception e)
{
System.err.println("不能读取属性文件. " +"请确保db.properties在CLASSPATH指定的路径中");
return;
}
String logFile = dbProps.getProperty("logfile", "DBConnectionManager.log");
try{
log = new PrintWriter(new FileWriter(logFile, true), true);
}
catch (IOException e)
{
System.err.println("无法打开日志文件: " + logFile);
log = new PrintWriter(System.err);
}
loadDrivers(dbProps);
createPools(dbProps); }catch(Exception e){}
}/**
* 171 * 装载和注册所有JDBC驱动程序 172 * 173 *
*
* @param props
* 属性 174
*/
private void loadDrivers(Properties props)
{
String driverClasses = props.getProperty("drivers");
StringTokenizer st = new StringTokenizer(driverClasses);
while (st.hasMoreElements())
{
String driverClassName = st.nextToken().trim();
try{
Driver driver = (Driver)
Class.forName(driverClassName).newInstance();
DriverManager.registerDriver(driver);
drivers.addElement(driver);
System.out.println(driverClassName);
log("成功注册JDBC驱动程序" + driverClassName);
}
catch (Exception e)
{
log("无法注册JDBC驱动程序: " +
driverClassName + ", 错误: " + e);
}
}
}/**
* 将文本信息写入日志文件
*/ public void log(String msg)
{
log.println(new Date() + ": " + msg);
}/**
* 将文本信息与异常写入日志文件
*/
public void log(Throwable e, String msg)
{
log.println(new Date() + ": " + msg);
e.printStackTrace(log);
}
}
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.Date;
import java.util.Enumeration;
import java.util.Vector;
class DBConnectionPool
{
//private int checkedOut;
private Vector freeConnections = new Vector();
private int maxConn;
private String name;
private String password;
private String URL;
private String user;
public PrintWriter log;
/**
* 创建新的连接池
*
* @param name 连接池名字
* @param URL 数据库的JDBC URL
* @param user 数据库帐号,或 null
* @param password 密码,或 null
* @param maxConn 此连接池允许建立的最大连接数
*/
public DBConnectionPool(String name, String URL, String user, String password,int maxConn)
{
this.name = name;
this.URL = URL;
this.user = user;
this.password = password;
this.maxConn = maxConn;
}
/**
* 将不再使用的连接返回给连接池
*
* @param con 客户程序释放的连接
*/
public synchronized void freeConnection(Connection con) {
// 将指定连接加入到向量末尾
try
{
if(con.isClosed()){
System.out.println("before freeConnection con is closed");
}
freeConnections.addElement(con);
Connection contest = (Connection) freeConnections.lastElement();
if(contest.isClosed()){
System.out.println("after freeConnection contest is closed");
}
notifyAll();
}catch(SQLException e){
System.out.println(e);
}
}
/**
* 从连接池获得一个可用连接.如没有空闲的连接且当前连接数小于最大连接
* 数限制,则创建新连接.如原来登记为可用的连接不再有效,则从向量删除之,
* 然后递归调用自己以尝试新的可用连接.
*/
public synchronized Connection getConnection()
{
Connection con = null;
if (freeConnections.size() > 0){
// 获取向量中第一个可用连接
con = (Connection) freeConnections.firstElement();
freeConnections.removeElementAt(0);
try {
if (con.isClosed())
{
log("从连接池" + name+"删除一个无效连接");
System.out.println("从连接池" + name+"删除一个无效连接");
// 递归调用自己,尝试再次获取可用连接
con = getConnection();
}
}
catch (SQLException e)
{
log("从连接池" + name+"删除一个无效连接时错误");
System.out.println("从连接池" + name+"删除一个无效连接出错");
// 递归调用自己,尝试再次获取可用连接
con = getConnection();
}
if(freeConnections.size()>maxConn)
{
System.out.println(" 删除一个溢出连接 ");
releaseOne();
}
}else if((maxConn == 0)||(freeConnections.size()<maxConn)){
con = newConnection();
}
return con;
}
public synchronized Connection returnConnection()
{
Connection con = null;
//如果闲置小于最大连接,返回一个新连接
if(freeConnections.size()<maxConn)
{
con = newConnection();
}
//如果闲置大于最大连接,返回一个可用的旧连接
else if(freeConnections.size()>=maxConn)
{
con = (Connection) freeConnections.firstElement();
System.out.println(" [a 连接池可用连接数 ] : "+"[ "+freeConnections.size()+" ]");
freeConnections.removeElementAt(0);
System.out.println(" [b 连接池可用连接数 ] : "+"[ "+freeConnections.size()+" ]");
try
{
if (con.isClosed())
{
log("从连接池" + name+"删除一个无效连接");
System.out.println("从连接池" + name+"删除一个无效连接");
returnConnection();
}
}catch (SQLException e)
{
log("从连接池" + name+"删除一个无效连接时错误");
System.out.println("从连接池" + name+"删除一个无效连接出错");
returnConnection();
}
}
return con;
}
/**
* 从连接池获取可用连接.可以指定客户程序能够等待的最长时间
* 参见前一个getConnection()方法.
*
* @param timeout 以毫秒计的等待时间限制
*/
public synchronized Connection getConnection(long timeout)
{
long startTime = new Date().getTime();
Connection con;
while ((con = getConnection()) == null)
{
try
{
wait(timeout);
}
catch (InterruptedException e) {}
if ((new Date().getTime() - startTime) >= timeout) {
// wait()返回的原因是超时
return null;
}
}
return con;
}
/**
* 关闭所有连接
*/
public synchronized void release()
{
Enumeration allConnections = freeConnections.elements();
while (allConnections.hasMoreElements())
{
Connection con = (Connection) allConnections.nextElement();
try {
con.close();
log("关闭连接池" + name+"中的一个连接");
}
catch (SQLException e)
{
log(e, "无法关闭连接池" + name+"中的连接");
}
}
freeConnections.removeAllElements();
}
/**
* 关闭一个连接
*/
public synchronized void releaseOne()
{
if(freeConnections.firstElement()!=null)
{ Connection con = (Connection) freeConnections.firstElement();
try {
con.close();
System.out.println("关闭连接池" + name+"中的一个连接");
log("关闭连接池" + name+"中的一个连接");
}
catch (SQLException e)
{
System.out.println("无法关闭连接池" + name+"中的一个连接");
log(e, "无法关闭连接池" + name+"中的连接");
}
}
else
{
System.out.println("releaseOne() bug.......................................................");
}
}
/**
* 创建新的连接
*/
private Connection newConnection()
{
Connection con = null;
try
{
if (user == null) {
con = DriverManager.getConnection(URL);
}
else{
con = DriverManager.getConnection(URL, user, password);
}
log("连接池" + name+"创建一个新的连接");
}
catch (SQLException e) {
log(e, "无法创建下列URL的连接: " + URL);
return null;
}
return con;
}
public void log(String msg)
{
log.println(new Date() + ": " + msg);
}/**
* 将文本信息与异常写入日志文件
*/
public void log(Throwable e, String msg)
{
log.println(new Date() + ": " + msg);
e.printStackTrace(log);
}
}
Connection[] num=new Connection[6];
第二行只是创建了num的6个引用对象,实际上并不是创建Connection的6个对象。
平时我们接触的String[]s = new String[6];其创建的String对象,其实是系统为其创建了默认了。但此时的Connection是有我们自己定义,因此你要自己创建对象,然后赋给num。
如果改为如下,就可以。
Connection[] num={new Connection(),new Connection(),new Connection(),new Connection(),new Connection(),new Connection()};