Java基础(五)-Java序列化与反序列化

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本文主要从以下方面记录：

1、Java序列化和反序列化是什么？

2、为什么需要序列化与反序列化？

3、怎么实现Java序列化和反序列化?

4、几个序列化注意事项

一、Java序列化和反序列化是什么？

　　通俗的来讲，序列化过程就是将对象转成二进制流存入内存或者文件，反序列化从内存或文件中读取二进制流转换成对象。

　　

二、为什么需要序列化与反序列化？

　　其实这个问题也就是它们的应用场景有哪些？这样就容易回答多了。比如文件（文本、图片等）进行传输，这些文件都是通过二进制序列的形式进行传输的（序列化过程），而接收方则要读取这些二进制数据进行相对应的转换（反序列化过程）。除了这个它主要用于网络传输（进程之间的通信等）。

三、怎么实现Java序列化和反序列化?

　　要想实现序列化有个必要条件就是要实现Serializable接口或Externalizable接口。大部分可能只知道有Serializable接口没有关注Externalizable接口，那么你看了本文之后就应该知道了，后面再介绍它们的区别。

　　有了上面个必要条件后还需要借助jdk中有两个类：java.io.ObjectOutputStream和java.io.ObjectInputStream，它们分别负责序列化和反序列化。我们可以看下这两个类的说明就知道是这两个类负责相对应的功能。

/**
 * An ObjectOutputStream writes primitive data types and graphs of Java objects
 * to an OutputStream.  The objects can be read (reconstituted) using an
 * ObjectInputStream.  Persistent storage of objects can be accomplished by
 * using a file for the stream.  If the stream is a network socket stream, the
 * objects can be reconstituted on another host or in another process.
 *
 * <p>Only objects that support the java.io.Serializable interface can be
 * written to streams.  The class of each serializable object is encoded
 * including the class name and signature of the class, the values of the
 * object's fields and arrays, and the closure of any other objects referenced
 * from the initial objects.
 * ...
 */
public class ObjectOutputStream
    extends OutputStream implements ObjectOutput, ObjectStreamConstants{}

/**
 * An ObjectInputStream deserializes primitive data and objects previously
 * written using an ObjectOutputStream.
 *
 * <p>ObjectOutputStream and ObjectInputStream can provide an application with
 * persistent storage for graphs of objects when used with a FileOutputStream
 * and FileInputStream respectively.  ObjectInputStream is used to recover
 * those objects previously serialized. Other uses include passing objects
 * between hosts using a socket stream or for marshaling and unmarshaling
 * arguments and parameters in a remote communication system.
 * ...
 */
public class ObjectInputStream
    extends InputStream implements ObjectInput, ObjectStreamConstants{
}    

　　下面我们以学生对象为例，将对象序列化保存至文件中，再从文件中反序列化转换成对象。

import java.io.Serializable;

/**
 * @Description: 学生类 已实现序列化接口
 * @author yuanfy
 * @date 2018年1月11日 上午11:36:37
 * @version 1.0
 */
public class Student implements Serializable{

    private static final long serialVersionUID = 6415983562512521049L;

    private String name;

    private int age;

    private String sex;

    public Student(String name, int age, String sex) {
        this.name = name;
        this.age = age;
        this.sex = sex;
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + ", sex=" + sex + "]";
    }
}

　　上面已经定义Java对象， 下面进行序列化测试。

    @Test
    public void testSerializable() throws FileNotFoundException, IOException{
        Student s = new Student("james", 31, "man");
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(new File("E:\\Student.txt")));
        oos.writeObject(s);
        oos.close();
    }

　　如果电脑E盘存在的话运行肯定通过（windows系统），单元测试通过后（说明序列化过程已完成）查看Student.txt文件中的内容，如下：

　　

　　上面是正常的情况， 如果Student类没有实现Serializable接口呢？那么序列化时会存在什么样的问题。我们把Student类去掉Serializable接口的实现，然后再进行序列化测试。这时你会发现单元测试后不通过，先看看报错原因：下面截图中的错误信息提示Student类没有被序列化。

　　

　　那么为什么java.io.ObjectOutputStream类会抛出这样的异常呢，它是怎么识别有没有实现Serializable接口的。接下来，我们来看看他的源码：writeObject方法中调用了writeObject()方法，所以主要逻辑在它里面。

    /**
     * Underlying writeObject/writeUnshared implementation.
     */
    private void writeObject0(Object obj, boolean unshared)
        throws IOException
    {
        boolean oldMode = bout.setBlockDataMode(false);
        depth++;
        try {
            //前面部分代码省略
            // remaining cases
            if (obj instanceof String) {
                writeString((String) obj, unshared);
            } else if (cl.isArray()) {
                writeArray(obj, desc, unshared);
            } else if (obj instanceof Enum) {
                writeEnum((Enum<?>) obj, desc, unshared);
            } else if (obj instanceof Serializable) {//判断是否是Serializable的子类。
                writeOrdinaryObject(obj, desc, unshared);
            } else {
                //除了String类型、数组类型和枚举类型，其他对象如果没有实现Serializable接口，都会抛出NotSerializableException异常
                if (extendedDebugInfo) {
                    throw new NotSerializableException(
                        cl.getName() + "\n" + debugInfoStack.toString());
                } else {
                    throw new NotSerializableException(cl.getName());
                }
            }
        } finally {
            depth--;
            bout.setBlockDataMode(oldMode);
        }
    }

　　从中可以看出，除了String类型、数组类型和枚举类型，其他对象如果没有实现Serializable接口，都会抛出NotSerializableException异常。

　　接下来进行反序列化测试：

    @Test
    public void testDeserialize() throws FileNotFoundException, IOException, ClassNotFoundException{
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File("E:\\Student.txt")));
        Student s = (Student)ois.readObject();
        System.out.println(s);
        ois.close();
    }

　　在进行正常的序列化测试后接着测试反序列化测试，正常来说是不会报错的。看看测试结果就明了了。

　　同样上面是正常的情况。接下来异常的情况就能体现出Student类中serialVersionUID变量的作用了：它就是验证序列化与反序列化的唯一性。在序列化后修改Student类中的serialVersionUID= 61234L,然后再测试反序列化，测试结果如下：

　　从错误提示中可以看出，会从解析出来的serialVersionUID和要转换的class中serialVersionUID做对比，判断是否相等。java.io.ObjectStreamClass关键源码如下：

    /**
     * Initializes class descriptor representing a non-proxy class.
     */
    void initNonProxy(ObjectStreamClass model,
                      Class<?> cl,
                      ClassNotFoundException resolveEx,
                      ObjectStreamClass superDesc)
        throws InvalidClassException
    {
        long suid = Long.valueOf(model.getSerialVersionUID());
        ObjectStreamClass osc = null;
        if (cl != null) {
            osc = lookup(cl, true);
            if (osc.isProxy) {
                throw new InvalidClassException(
                        "cannot bind non-proxy descriptor to a proxy class");
            }
            if (model.isEnum != osc.isEnum) {
                throw new InvalidClassException(model.isEnum ?
                        "cannot bind enum descriptor to a non-enum class" :
                        "cannot bind non-enum descriptor to an enum class");
            }
            //这里会判断serialVersionUID是否一致
            if (model.serializable == osc.serializable &&
                    !cl.isArray() &&
                    suid != osc.getSerialVersionUID()) {
                throw new InvalidClassException(osc.name,
                        "local class incompatible: " +
                                "stream classdesc serialVersionUID = " + suid +
                                ", local class serialVersionUID = " +
                                osc.getSerialVersionUID());
            }
            //后面代码略
        }
    }

四、几个序列化注意事项

　　1、对继承父类的子类序列化

public class Person {
    public int age;

    public int weight;

    public Person(int age, int weight) {
        this.age = age;
        this.weight = weight;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getWeight() {
        return weight;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }

}

import java.io.Serializable;

public class Women extends Person implements Serializable{

    private static final long serialVersionUID = -1259423203325949704L;

    private String name;

    public Women(String name, int age, int weight) {
        super(age, weight);
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + ", weight=" + weight + "]";
    }
}

　　单元测试代码如下，先猜错下会不会运行通过，结果又是啥？

    @Test
    public void test2() throws FileNotFoundException, IOException, ClassNotFoundException{
        Women w = new Women("Scarlett Johansson", 34, 50);
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(new File("E:\\Women.txt")));
        oos.writeObject(w);
        oos.close();

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File("E:\\Women.txt")));
        Women s = (Women)ois.readObject();
        System.out.println(s);
        ois.close();
    }

　　测试结果如下，居然报错。该错误提示需要提供无参构造函数(During deserialization, the fields of non-serializable classes will be initialized using the public or protected no-arg constructor of the class. A no-arg constructor must be accessible to the subclass that is serializable. The fields of serializable subclasses will be restored from the stream.)，具体详情解释见这here.

　　

　　当我们提供无参构造函数后再进行单元测试得到结果：Student [name=Scarlett Johansson, age=0, weight=0]。所以可以得出结论：父类中的字段不参与序列化，只是将其初始化而已。

　　2、transient 关键字

　　使用transient关键字来修饰变量，然后进行序列化（在实现serializable接口情况下）效果其实和父类的序列化一样，它所修饰的变量不参与序列化。这里就不举例说明了，自己可以写个案例测试下。

　　另外transient关键字只能修饰变量， 不能修饰类和方法。

　　3、static关键字

　　使用static关键字来修饰变量，不管有没有transient修饰，同样不参与序列化（在实现serializable接口情况下）。

　　4、Externalizable接口

　　 Externalizable接口extends Serializable接口，而且在其基础上增加了两个方法：writeExternal()和readExternal()。这两个方法会在序列化和反序列化还原的过程中被自动调用，以便执行一些特殊的操作。

　　下面看案例：

import java.io.Externalizable;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectOutput;

/**
 * @Description: 学生类 已实现序列化接口
 * @author yuanfy
 * @date 2018年1月11日 上午11:36:37
 * @version 1.0
 */
public class Student implements Externalizable{

    private static final long serialVersionUID = 61234L;

    private String name;

    private transient int age;

    private static String sex;

    //如果覆盖了无参构造函数就一定要显示声明无参构造函数，跟父类序列化的案例一样。
    public Student(){}

    public Student(String name, int age, String sex) {
        this.name = name;
        this.age = age;
        this.sex = sex;
    }

    public  String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + ", sex=" + sex + "]";
    }

    @Override
    public void writeExternal(ObjectOutput out) throws IOException {
        // TODO Auto-generated method stub
        out.writeUTF(name);
        out.writeInt(age);
        out.writeObject(sex);
    }

    @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        this.name = in.readUTF();
        this.age = in.readInt();
        this.sex = (String)in.readObject();
    }
}

    @Test
    public void test3() throws FileNotFoundException, IOException, ClassNotFoundException{
        Student s1 = new Student("james", 31, "man");
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(new File("E:\\Student.txt")));
        oos.writeObject(s1);
        oos.close();

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File("E:\\Student.txt")));
        Student s2 = (Student)ois.readObject();
        System.out.println(s2);
        ois.close();
    }

　　上面是正常完整案例，其中要注意的是，要序列化的类要提供无参构造函数，否则反序列化会报错（When an Externalizable object is reconstructed, an instance is created using the public no-arg constructor, then the readExternal method called. Serializable objects are restored by reading them from an ObjectInputStream.）详解here。得出结论如下：

　　与Serizable对象不同，使用Externalizabled，就意味着没有任何东西可以自动序列化， 为了正常的运行，我们需要在writeExtenal()方法中将自对象的重要信息写入，从而手动的完成序列化。对于一个Externalizabled对象，对象的默认构造函数都会被调用（包括哪些在定义时已经初始化的字段），然后调用readExternal()，在此方法中必须手动的恢复数据。这就说明在Externalizable接口下不管是transient或static修饰的变量，如果没有指定写入，就不会序列化。