关于本系列

信息存储和检索作为同义语伴随 RDBMS 已经有 10 余年了，但现在情况有所改变。Java 开发人员为所谓的对象关系型阻抗失配而沮丧，也不再有耐心去尝试解决这个问题。加上可行替代方案的出现，就导致了人们对对象持久性和检索的兴趣的复苏。面向 Java 开发人员的 db4o 指南 对开放源码数据库 db4o 进行了介绍，db4o 可以充分利用当前的面向对象的语言、系统和理念。请访问 db4o 主页，立即下载 db4o。您需要用它来完成本文中的例子。

package com.tedneward.model;

public class Person
{
    public Person()
    { }
    public Person(String firstName, String lastName, int age)
    {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }
    
    public String getFirstName() { return firstName; }
    public void setFirstName(String value) { firstName = value; }
    
    public String getLastName() { return lastName; }
    public void setLastName(String value) { lastName = value; }
    
    public int getAge() { return age; }
    public void setAge(int value) { age = value; }

    public String toString()
    {
        return 
            "[Person: " +
            "firstName = " + firstName + " " +
            "lastName = " + lastName + " " +
            "age = " + age + 
            "]";
    }
    
    public boolean equals(Object rhs)
    {
        if (rhs == this)
            return true;
        
        if (!(rhs instanceof Person))
            return false;
        
        Person other = (Person)rhs;
        return (this.firstName.equals(other.firstName) &&
                this.lastName.equals(other.lastName) &&
                this.age == other.age);
    }
    
    private String firstName;
    private String lastName;
    private int age;
}

import java.io.*;
import java.lang.reflect.*;
import com.db4o.*;
import com.tedneward.model.*;

// Version 1
public class BuildV1
{
    public static void main(String[] args)
        throws Exception
    {
        new File(".", "persons.data").delete();
        
        ObjectContainer db = null;
        try
        {
            db = Db4o.openFile("persons.data");

            Person brianG = new Person("Brian", "Goetz", 39);
            Person jason = new Person("Jason", "Hunter", 35);
            Person brianS = new Person("Brian", "Sletten", 38);
            Person david = new Person("David", "Geary", 55);
            Person glenn = new Person("Glenn", "Vanderberg", 40);
            Person neal = new Person("Neal", "Ford", 39);
            Person clinton = new Person("Clinton", "Begin", 19);
            
            db.set(brianG);
            db.set(jason);
            db.set(brianS);
            db.set(david);
            db.set(glenn);
            db.set(neal);
            db.set(clinton);

            db.commit();
            
            // Find all the Brians
            ObjectSet brians = db.get(new Person("Brian", null, 0));
            while (brians.hasNext())
                System.out.println(brians.next());            
        }
        finally
        {
            if (db != null)
                db.close(); 
        }
    }
}

package com.tedneward.model;

public enum Mood
{
    HAPPY, CONTENT, BLAH, CRANKY, DEPRESSED, PSYCHOTIC, WRITING_AN_ARTICLE
}

package com.tedneward.model;

// Person v2
public class Person
{
    // ... as before, with appropriate modifications to public constructor and
    // toString() method
    
    public Mood getMood() { return mood; }
    public void setMood(Mood value) { mood = value; }

    private Mood mood;
}

import com.db4o.*;
import com.tedneward.model.*;

// Version 2
public class ReadV2
{
    public static void main(String[] args)
        throws Exception
    { // Note the absence of the File.delete() call ObjectContainer db = null;
        try
        {
            db = Db4o.openFile("persons.data");

            // Find all the Brians
            ObjectSet brians = db.get(new Person("Brian", null, 0, null));
            while (brians.hasNext())
                System.out.println(brians.next());            
        }
        finally
        {
            if (db != null)
                db.close(); 
        }
    }
}

[Person: firstName = Brian lastName = Sletten age = 38 mood = null]
[Person: firstName = Brian lastName = Goetz age = 39 mood = null]

import com.db4o.*;
import com.tedneward.model.*;

// Version 2
public class BuildV2
{
    public static void main(String[] args)
        throws Exception
    {
        ObjectContainer db = null;
        try
        {
            db = Db4o.openFile("persons.data");

            // Find all the Persons, and give them moods
            ObjectSet people = db.get(Person.class);
            while (people.hasNext())
            {
                Person person = (Person)people.next();
                
                System.out.print("Setting " + person.getFirstName() + "'s mood ");
                int moodVal = (int)(Math.random() * Mood.values().length);
                person.setMood(Mood.values()[moodVal]);
                System.out.println("to " + person.getMood());
                db.set(person);
            }
            
            db.commit();            
        }
        finally
        {
            if (db != null)
                db.close(); 
        }
    }
}

Setting Brian's mood to BLAH
Setting David's mood to WRITING_AN_ARTICLE
Setting Brian's mood to CONTENT
Setting Jason's mood to PSYCHOTIC
Setting Glenn's mood to BLAH
Setting Neal's mood to HAPPY
Setting Clinton's mood to DEPRESSED

[Person: firstName = Brian lastName = Sletten age = 38]
[Person: firstName = Brian lastName = Goetz age = 39]

db4o 保存旧的数据吗？

如果删除一个字段，然后再添加那个字段，db4o 竟然可以找回那个字段当初存在时原有的值。不，db4o 不会永远跟踪所有被删除的字段的值 —— 当数据库被请求执行所谓的碎片整理操作时，它就会删除那些值。本系列将来的文章中会更详细地谈到碎片整理，请继续关注。

import com.db4o.*;
import com.db4o.config.*;

// ...

Db4o.configure().objectClass("com.tedneward.model.Person")
    .rename("com.tedneward.persons.model.Individual");

import com.db4o.config.*;

// ...

TypeAlias fromPersonToIndividual = 
    new TypeAlias("com.tedneward.model.Person", "com.tedneward.persons.model.Individual");
Db4o.configure().addAlias(fromPersonToIndividual);

更多重构方法

我还没有谈到 db4o 支持重构的所有方法，也就是说还有很多要学的东西。即使您发现 db4o 的重构选项不能很好地处理自己的情况，也仍然有旧的后备选项可用，您可以在要求的位置用一个临时名称创建新类，编写一些代码从旧类创建新类的对象，然后删除旧的对象，并将临时类重新命名为适当的名称。如果急于知道这种选项，请参阅 db4o 的 doc\reference directory 的 Advanced Type Handling 小节中的 “Refactoring and meta-information”。