Java对象序列化

序列化是一种将Java对象图转换为用于存储(到磁盘文件)或传输(通过网络)的字节数组的机制，然后通过使用反序列化，我们可以恢复对象图。 使用引用共享机制正确地恢复对象的图。但是在存储之前，请检查input-file/network中的serialVersionUID和.class文件中的serialVersionUID是否相同。如果不是，则抛出java.io.InvalidClassException。

每个版本化的类必须确定它能够为其写入流和从中读取流的原始类版本。例如，一个有版本控制的类必须声明: serialVersionUID的语法 // ANY-ACCESS-MODIFIER static final long serialVersionUID = (64-bit has)L; private static final long serialVersionUID = 3487495895819393L;

serialVersionUID是序列化过程所必需的。但是开发人员可以选择将其添加到java源文件中。如果没有包含serialVersionUID，序列化运行时将生成serialVersionUID并将其与类关联。序列化对象将包含这个serialVersionUID以及其他数据。

注意——强烈建议所有可序列化类显式声明serialVersionUID，因为默认的serialVersionUID计算对类细节非常敏感，这些细节可能会根据编译器实现的不同而不同，因此可能会在反序列化期间导致意外的serialVersionUID冲突，导致反序列化失败。

检查可序列化类

Java对象只能序列化。如果一个类或它的任何超类实现了java.io.Serializable接口 或其子接口java.io.Externalizable。

A class must implement java.io.Serializable interface in order to serialize its object successfully. Serializable is a marker interface and used to inform the compiler that the class implementing it has to be added serializable behavior. Here Java Virtual Machine (JVM) is responsible for its automatic serialization. transient Keyword: java.io.Serializable interface While serializing an object, if we don't want certain data members of the object to be serialized we can use the transient modifier. The transient keyword will prevent that data member from being serialized. Fields declared as transient or static are ignored by the serialization process. TRANSIENT & VOLATILE +--------------+--------+-------------------------------------+ | Flag Name | Value | Interpretation | +--------------+--------+-------------------------------------+ | ACC_VOLATILE | 0x0040 | Declared volatile; cannot be cached.| +--------------+--------+-------------------------------------+ |ACC_TRANSIENT | 0x0080 | Declared transient; not written or | | | | read by a persistent object manager.| +--------------+--------+-------------------------------------+ class Employee implements Serializable { private static final long serialVersionUID = 2L; static int id; int eno; String name; transient String password; // Using transient keyword means its not going to be Serialized. } Implementing the Externalizable interface allows the object to assume complete control over the contents and format of the object's serialized form. The methods of the Externalizable interface, writeExternal and readExternal, are called to save and restore the objects state. When implemented by a class they can write and read their own state using all of the methods of ObjectOutput and ObjectInput. It is the responsibility of the objects to handle any versioning that occurs. class Emp implements Externalizable { int eno; String name; transient String password; // No use of transient, we need to take care of write and read. @Override public void writeExternal(ObjectOutput out) throws IOException { out.writeInt(eno); out.writeUTF(name); //out.writeUTF(password); } @Override public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { this.eno = in.readInt(); this.name = in.readUTF(); //this.password = in.readUTF(); // java.io.EOFException } } Only objects that support the java.io.Serializable or java.io.Externalizable interface can be written to/read from 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.

文件的序列化示例

public class SerializationDemo {
    static String fileName = "D:/serializable_file.ser";

    public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException {
        Employee emp = new Employee( );
        Employee.id = 1; // Can not Serialize Class data.
        emp.eno = 77;
        emp.name = "Yash";
        emp.password = "confidential";
        objects_WriteRead(emp, fileName);

        Emp e = new Emp( );
        e.eno = 77;
        e.name = "Yash";
        e.password = "confidential";
        objects_WriteRead_External(e, fileName);

        /*String stubHost = "127.0.0.1";
        Integer anyFreePort = 7777;
        socketRead(anyFreePort); //Thread1
        socketWrite(emp, stubHost, anyFreePort); //Thread2*/

    }
    public static void objects_WriteRead( Employee obj, String serFilename ) throws IOException{
        FileOutputStream fos = new FileOutputStream( new File( serFilename ) );
        ObjectOutputStream objectOut = new ObjectOutputStream( fos );
        objectOut.writeObject( obj );
        objectOut.close();
        fos.close();

        System.out.println("Data Stored in to a file");

        try {
            FileInputStream fis = new FileInputStream( new File( serFilename ) );
            ObjectInputStream ois = new ObjectInputStream( fis );
            Object readObject;
            readObject = ois.readObject();
            String calssName = readObject.getClass().getName();
            System.out.println("Restoring Class Name : "+ calssName); // InvalidClassException

            Employee emp = (Employee) readObject;
            System.out.format("Obj[No:%s, Name:%s, Pass:%s]", emp.eno, emp.name, emp.password);

            ois.close();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
    public static void objects_WriteRead_External( Emp obj, String serFilename ) throws IOException {
        FileOutputStream fos = new FileOutputStream(new File( serFilename ));
        ObjectOutputStream objectOut = new ObjectOutputStream( fos );

        obj.writeExternal( objectOut );
        objectOut.flush();

        fos.close();

        System.out.println("Data Stored in to a file");

        try {
            // create a new instance and read the assign the contents from stream.
            Emp emp = new Emp();

            FileInputStream fis = new FileInputStream(new File( serFilename ));
            ObjectInputStream ois = new ObjectInputStream( fis );

            emp.readExternal(ois);

            System.out.format("Obj[No:%s, Name:%s, Pass:%s]", emp.eno, emp.name, emp.password);

            ois.close();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

可通过网络序列化的例子

将对象的状态分布在不同的地址空间中，或者在同一台计算机上的不同进程中，或者甚至在通过网络连接的多台计算机中，但它们通过共享数据和调用方法一起工作。

数据编组 残根和骷髅

/**
 * Creates a stream socket and connects it to the specified port number on the named host. 
 */
public static void socketWrite(Employee objectToSend, String stubHost, Integer anyFreePort) {
    try { // CLIENT - Stub[marshalling]
        Socket client = new Socket(stubHost, anyFreePort);
        ObjectOutputStream out = new ObjectOutputStream(client.getOutputStream());
        out.writeObject(objectToSend);
        out.flush();
        client.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}
// Creates a server socket, bound to the specified port. 
public static void socketRead(  Integer anyFreePort ) {
    try { // SERVER - Stub[unmarshalling ]
        ServerSocket serverSocket = new ServerSocket( anyFreePort );
        System.out.println("Server serves on port and waiting for a client to communicate");
            /*System.in.read();
            System.in.read();*/

        Socket socket = serverSocket.accept();
        System.out.println("Client request to communicate on port server accepts it.");

        ObjectInputStream in = new ObjectInputStream(socket.getInputStream());
        Employee objectReceived = (Employee) in.readObject();
        System.out.println("Server Obj : "+ objectReceived.name );

        socket.close();
        serverSocket.close();
    } catch (IOException | ClassNotFoundException e) {
        e.printStackTrace();
    }
}

@see

实现Serializable和Externalizable

序列化是将对象转换为一系列字节，以便可以轻松地将对象保存到持久存储器或通过通信链路进行流式传输。然后可以反序列化字节流，将其转换为原始对象的副本。

我将提供一个类比，以潜在地帮助巩固对象序列化/反序列化的概念目的/实用性。

I imagine object serialization/deserialization in the context of attempting to move an object through a storm drain. The object is essentially "decomposed" or serialized into more modular versions of itself - in this case, a series of bytes - in order to effectively be granted passage through a medium. In a computational sense, we could view the path traveled by the bytes through the storm drain as being akin to bytes traveling through a network. We're transmuting our object in order to conform to a more desirable mode of transportation, or format. The serialized object will typically be stored in a binary file which may later be read from, written to, or both.

也许一旦我们的对象能够作为分解的字节序列通过drain，我们可能希望将对象的表示形式作为二进制数据存储在数据库或硬盘驱动器中。不过，主要的要点是，通过序列化/反序列化，我们可以选择让对象在序列化后保持二进制形式，或者通过执行反序列化来“检索”对象的原始形式。

Serialization is the process of saving an object in a storage medium (such as a file, or a memory buffer) or to transmit it over a network connection in binary form. The serialized objects are JVM independent and can be re-serialized by any JVM. In this case the "in memory" java objects state are converted into a byte stream. This type of the file can not be understood by the user. It is a special types of object i.e. reused by the JVM (Java Virtual Machine). This process of serializing an object is also called deflating or marshalling an object.

要序列化的对象必须实现java.io.Serializable Interface。 对象的默认序列化机制写入对象的类、类签名以及所有非瞬态和非静态字段的值。

class ObjectOutputStream extends java.io.OutputStream implements ObjectOutput,

ObjectOutput接口扩展了DataOutput接口，并添加了用于序列化对象和向文件写入字节的方法。ObjectOutputStream扩展了java.io.OutputStream并实现了ObjectOutput接口。它将对象、数组和其他值序列化到流中。因此ObjectOutputStream的构造函数被写成:

ObjectOutput ObjOut = new ObjectOutputStream(new FileOutputStream(f));

上面的代码已用于使用ObjectOutputStream()构造函数创建ObjectOutput类的实例，该构造函数将FileOuputStream的实例作为参数。

ObjectOutput接口用于实现ObjectOutputStream类。构造ObjectOutputStream是为了序列化对象。

在java中反序列化对象

与序列化相反的操作称为反序列化，即从一系列字节中提取数据称为反序列化，也称为膨胀或解组。

ObjectInputStream扩展了java.io.InputStream，实现了ObjectInput接口。它反序列化来自输入流的对象、数组和其他值。因此ObjectInputStream的构造函数被写成:

ObjectInputStream obj = new ObjectInputStream(new FileInputStream(f));

上面的程序代码创建了ObjectInputStream类的实例，以反序列化已被ObjectInputStream类序列化的文件。上面的代码使用FileInputStream类的实例创建实例，FileInputStream类包含必须反序列化的指定文件对象，因为ObjectInputStream()构造函数需要输入流。

序列化是通过将对象转换为字节码来保存对象的特定状态的过程。转换后的字节码用于在2个JVM之间传输对象状态，接收JVM在其中反序列化字节码以检索共享对象的状态。 序列化和反序列化是使用serialVersionUID作为相关jvm中的引用来完成的。

在Java中，使用Serialization和Externalisation接口也可以实现同样的功能

序列化意味着在java中持久化对象。如果您希望保存对象的状态，并希望稍后重新构建该状态(可能在另一个JVM中)，则可以使用序列化。

注意，对象的属性只会被保存。如果你想再次恢复对象，你应该有类文件，因为只存储成员变量而不存储成员函数。

eg:

ObjectInputStream oos = new ObjectInputStream(                                 
                                 new FileInputStream(  new File("o.ser")) ) ;
SerializationSample SS = (SearializationSample) oos.readObject();

serializable是一个标记接口，用来标记你的类是可序列化的。标记接口意味着它只是一个空接口，使用该接口将通知JVM该类可以被序列化。