[转载]EJB 入门：Bean-Managed Persistence(BMP)

dinner1007
2019-05-16 10:27:05
Linux操作系统
原创

EJB 入门

Bean-Managed Persistence(BMP)

本章讲述内容

Bean-Managed 和 Container-Managed persistence 的区别
如何实现 Bean-managed persistence 的实体 Bean

Bean-Managed Persistence

容器管理 Bean 的生命周期－－何时创建、载入、存储和删除
开发人员处理其机制－－如何创建、载入、存储和删除
BMP = [业务逻辑] + [存储逻辑]

为何使用 BMP?

为了存储 CMP 现阶段所不能处理的复杂的 Bean
包含如下数据的实体 Bean:
- 内嵌 Java 对象
- 对象的集合
- 对其它 Bean 的引用
- ...
CMP 所不支持的数据源

BMP 将您紧紧地绑定到某个特定的存储机制上－－可能将您限制于一个特定的平台！

在 Home 接口的 Create 方法在 Bean 实例中有对应的 ejbCreate 方法－－ejbCreate 方法返回一个主键对象
可以有任意数目的 create 方法

public ClientKey ejbCreate(int id, String first, String last) 
throws CreateException, RemoteException {
    if (new File("client." + id).exists())
        throw new CreateException();
    setFirstName(first);
    setLastName(last);
    setPhoneNumber(null);
    return new ClientKey(id);
}

ejbStore() 方法完成工作－－entityContext 包含主键

public void ejbStore() throws RemoteException {
    try {
        ObjectOutputStream out 
            = new ObjectOutputStream(new FileOutputStream("client." + getId()));
        out.writeObject(getFirstName());
        out.writeObject(getLastName());
        out.writeObject(getPhoneNumber());
        out.close();
    } catch (IOException e) {
        throw new RemoteException("An IO error occurred", e);
    }   
}

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

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();
}

public void ejbStore() throws RemoteException {
    try {
        PrintWriter out 
= new PrintWriter(new FileWriter("client." + getId()));
        out.println("
");
        out.print("        out.print(getId());
        out.print("">
	");
        out.print(getFirstName());
        out.print("
	");
        out.print(getLastName());
        out.print("
	");
        if (getPhoneNumber() != null) out.print(getPhoneNumber());
        out.println("
");
        out.close();
    } catch (IOException e) {
        e.printStackTrace();
        throw new RemoteException("An IO error occurred", e);
    }   
}



    Kenneth
    Beaton
    555-9999

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

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);
}

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        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

Bean-Managed Persistence 允许 Bean 开发人员指定如何实现数据的存储
ejbLoad, ejbStore 和 ejbRemove 方法必需先从 entityContext 中获得主键
BMP create 方法必需返回主键
对应每个在 Home 接口中的 find* 方法必需有一个对应的 ejbFind* 方法－－开发人员必需至少提供 ejbFindByPrimaryKey