[size=large]以下是孙鑫老师书上的代码有几个地方不明白。[/size]
[code="java"]
package org.sunxin.ch09.bookstore;
import java.io.Serializable;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.sql.DataSource;
public class BookDBBean implements Serializable {
private DataSource ds = null;
public BookDBBean() throws NamingException {
Context ctx = new InitialContext();
ds = (DataSource)ctx.lookup("java:comp/env/jdbc/bookstore");
}
public Connection getConnection() throws SQLException {
return ds.getConnection();
}
protected void closeConnection(Connection conn) {
if(conn!=null) {
try {
conn.close();
conn = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
}
protected void closeStatement(Statement stmt) {
if(stmt != null) {
try {
stmt.close();
stmt = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
}
protected void closePreparedStatement(PreparedStatement pstmt) {
if(pstmt != null) {
try {
pstmt.close();
pstmt = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
}
protected void closeResultSet(ResultSet rs) {
if(rs != null) {
try {
rs.close();
rs = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
}
public Collection<BookBean> getBooks() throws SQLException {
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
try {
Collection<BookBean> bookList = new ArrayList<BookBean>();
conn = this.getConnection();
stmt = conn.createStatement();
rs = stmt.executeQuery("select * from bookinfo");
while(rs.next()) {
BookBean book = new BookBean(rs.getInt(1), rs.getString(2), rs.getString(3), rs.getString(4),
rs.getString(5), rs.getFloat(6), rs.getInt(7), rs.getString(8));
bookList.add(book);
}
return bookList;
} finally {
closeResultSet(rs);
closeStatement(stmt);
closeConnection(conn);
}
}
public BookBean getBook(int bookId) throws SQLException {
Connection conn = null;
PreparedStatement pstmt = null;
ResultSet rs = null;
try {
conn = getConnection();
pstmt = conn.prepareStatement("select * from bookinfo where bookId = ?");
pstmt.setInt(1, bookId);
rs = pstmt.executeQuery();
BookBean book = null;
if(rs.next()) {
book = new BookBean(rs.getInt(1), rs.getString(2), rs.getString(3), rs.getString(4),
rs.getString(5), rs.getFloat(6), rs.getInt(7), rs.getString(8));
}
return book;
} finally {
closeResultSet(rs);
closePreparedStatement(pstmt);
closeConnection(conn);
}
}
public Collection<BookBean> searchBook(String keyword)throws SQLException {
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
ArrayList<BookBean> bookList = new ArrayList<BookBean>();
try {
conn = getConnection();
stmt = conn.createStatement();
String sql = "select * from bookinfo where title like " + "'%" + keyword + "%'";
rs = stmt.executeQuery(sql);
while(rs.next()) {
BookBean book = new BookBean(rs.getInt(1), rs.getString(2), rs.getString(3), rs.getString(4),
rs.getString(5), rs.getFloat(6), rs.getInt(7), rs.getString(8));
bookList.add(book);
}
return bookList;
} finally {
closeResultSet(rs);
closeStatement(stmt);
closeConnection(conn);
}
}
public boolean isAmountEnough(int bookId, int quantity) throws SQLException{
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
boolean bEnough = false;
try {
conn = getConnection();
stmt = conn.createStatement();
String sql = "select amount from bookinfo where id = " + bookId;
rs = stmt.executeQuery(sql);
if(rs.next()) {
int amount = rs.getInt(1);
if(amount >= quantity) {
bEnough = true;
}
}
return bEnough;
} finally {
closeResultSet(rs);
closeStatement(stmt);
closeConnection(conn);
}
}
public synchronized void buyBooks(CartBean cart) throws SQLException{
Connection conn = null;
PreparedStatement pstmt = null;
Iterator<CartItemBean> it = cart.getItems().iterator();
try {
conn = this.getConnection();
String sql = "update bookinfo set amount = amount - ? where id = ?";
pstmt = conn.prepareStatement(sql);
while(it.hasNext()) {
CartItemBean item = it.next();
BookBean book = item.getBook();
int bookId = book.getId();
int quantity = item.getQuantity();
pstmt.setInt(1, quantity);
pstmt.setInt(2, bookId);
pstmt.addBatch();
}
pstmt.executeBatch();
} finally {
this.closePreparedStatement(pstmt);
this.closeConnection(conn);
}
}
}
[/code]
[size=large][color=red]
1.为何不定义成员变量 private Connection conn;呢？ 是出于安全考虑吗？
2.为何在关闭连接的处理中对异常做了处理，而在其他的方法中却要将异常抛出呢？
抛出有什么好处呢？(这个问题请将详细些吧。琢磨好长时间了。异常基础知识看了一遍又一遍，还是不理解为什么这样用)
[/color]
[quote]1.为何不定义成员变量 private Connection conn;呢？ 是出于安全考虑吗？[/quote]
关于这一点，是为了线程安全。当系统里面有多个请求同时进行的时候，要保证使用的不是同一个连接，这样不会影响到事务处理。
[quote]2.为何在关闭连接的处理中对异常做了处理，而在其他的方法中却要将异常抛出呢？
抛出有什么好处呢？(这个问题请将详细些吧。琢磨好长时间了。异常基础知识看了一遍又一遍，还是不理解为什么这样用) [/quote]
关闭连接的异常对于服务的使用者来说是没有必要知道的。把信息返回没有任何的价值，所以直接在内部消化掉。
但是其他方法中的数据库CRUD操作失败的情况是需要返回给使用者的。所以要向上层抛出，最后由UI层统一处理掉。
buaalijie 回答的很清楚了