【标题】：EJB（Enterprise JavaBeans）入门（9）
【关键字】：Java,is,EJB,Bean,Enterprise,JavaBean,Beans,Be,EJB,Enterprise,JavaBeans
【来源】：网络

EJB（Enterprise JavaBeans）入门（9）

private int getId() {
return ((ClientKey)getEntityContext().getPrimaryKey()).id;
}

· 载入

在进行载入前，entityContext 将被设置－－entityContext 包含主键
使用主键的值来查找和载入 Bean

public void ejbLoad() throws RemoteException {
try {
ObjectInputStream in
= new ObjectInputStream(new FileInputStream("client." + getId()));
setFirstName((String)in.readObject());
setLastName((String)in.readObject());
setPhoneNumber((String)in.readObject());
in.close();
} catch (ClassNotFoundException e) {
// Shouldn't happen -- We're dealing with Strings here.
} catch (IOException e) {
throw new RemoteException("An IO error occurred", e);
}
}

· 删除 ejbRemove() 方法用来完成删除工作

public void ejbRemove() throws RemoteException, RemoveException {
File file = new File("client." + getId());
if (!file.exists()) throw new RemoveException();
file.delete();
}

· XML 示例

public void ejbStore() throws RemoteException {
try {
PrintWriter out
= new PrintWriter(new FileWriter("client." + getId()));
out.println("<?xml version=\"1.0\" ?>\n");
out.print("<client id=\"");
out.print(getId());
out.print("\">\n\t<first>");
out.print(getFirstName());
out.print("</first>\n\t<last>");
out.print(getLastName());
out.print("</last>\n\t<phone>");
if (getPhoneNumber() != null) out.print(getPhoneNumber());
out.println("</phone>\n</client>");
out.close();
} catch (IOException e) {
e.printStackTrace();
throw new RemoteException("An IO error occurred", e);
}
}

<?xml version="1.0" ?>

<client id="999999">
<first>Kenneth</first>
<last>Beaton</last>
<phone>555-9999</phone>
</client>

· Finder 方法

对每个在 Home 接口中声明的 find 方法必需有一个对应的 ejbFind 方法
单个查找－－返回的是找到对象的主键
多个查找－－返回的是匹配对象的主键的枚举
容器将载入 Bean－－一个 Bean 从池中取出，赋以主键(通过 entityContext)然后被调用载入方法

· 单个查找

public ClientKey ejbFindByPrimaryKey(ClientKey key)
throws RemoteException, FinderException {
if (!new File("client." + key.id).exists())
throw new FinderException();
return key;
}

public ClientKey ejbFindById(int id)
throws RemoteException, FinderException {
if (!new File("client." + id).exists())
throw new FinderException();
return new ClientKey(id);
}

· 多个查找

public Enumeration ejbFindAll()
throws RemoteException, FinderException {
String[] files = new File(".").list(new FilenameFilter() {
public boolean accept(File directory, String name) {
return name.startsWith("client.");
}
});

Vector keys = new Vector();
for (int index=0;index<files.length;index++) {
String name = files[index];
int id=Integer.parseInt(name.substring(name.indexOf('.') + 1));
keys.addElement(new ClientKey(id));
}

return keys.elements();
}

· 多个查找 (JDBC 版本)

public Enumeration ejbFindAll() throws RemoteException, FinderException {
Vector keys = new Vector();
Connection connection = null;
PreparedStatement statement = null;
try {
connection = getDataSource().getConnection();
statement = connection.prepareStatement("Select ID from CLIENT");
ResultSet results = statement.executeQuery();
while (results.next()) {
int id = results.getInt(1);
keys.addElement(new ClientKey(id));
}
results.close();
} catch (SQLException e) {
} finally {
try {statement.close();} catch (SQLException e) { }
try {connection.close();} catch (SQLException e) {}
}
return keys.elements();
}

· JDBC 载入示例

public void ejbLoad() throws RemoteException {
Connection connection = null;
PreparedStatement statement = null;
try {
connection = getDataSource().getConnection();
statement = connection
.prepareStatement("Select FIRST, LAST, PHONE from CLIENT where ID=?");
statement.setInt(1, getId());
ResultSet results = statement.executeQuery();
if (results.next()) {
setFirstName(results.getString(1));
setLastName(results.getString(2));
setPhoneNumber(results.getString(3));
} else {
throw new RemoteException("The row cannot be found!");
}
results.close();
} catch (SQLException e) {
...
}

· 查找的过程

有一个 Bean 实例被用来查找出其它实例－－Bean 必需被从池中取出来进行查找工作
Bean 实例必需自己将自己载入－－可能开销会很大
使用 JDBC:
- 一个调用用来取出所有您所关心的 Bean 的 id
- 对每个 id 又是一个调用来取出一行数据
- 总共需要 n+1 个对数据库的访问来载入 n 个 Bean!

· 本章讲述内容