[Chapter 11] Externalizable

#

| Chapter 11 The java.io Package |

# Externalizable

## Name

Externalizable

## Synopsis

Interface Name:

  `java.io.Externalizable`

Super-interface:

  `java.io.Serializable`

Immediate Sub-interfaces:

  None

Implemented By:

  None

Availability:

  New as of JDK 1.1

## Description

The `Externalizable` interface is an extension of the `Serializable` interface. Whereas a `Serializable` object is automatically saved and loaded (in most cases), an `Externalizable` object has sole responsibility for saving and loading its state via the `writeExternal()` and `readExternal()` methods. If a class implements the `Externalizable` interface, it must handle any versioning issues that occur.

The methods of `Externalizable` are `public`, which can pose a security risk. If security is a concern, `Externalizable` objects should not write or read sensitive information, or the `Serializable` interface should be used instead.

## Interface Declaration

    public abstract interface java.io.Externalizable
                              extends java.io.Serializable {
      // Methods
      public abstract void readExternal(ObjectInput in);
      public abstract void writeExternal(ObjectOutput out);
    }

## Methods

### readExternal

####

     public abstract void readExternal(ObjectInput in) throws IOException, ClassNotFoundException

Parameters

  `in`

    The object input stream to use.

Throws

  `ClassNotFoundException`

    If the class of the object being deserialized cannot be found.

  `IOException`

    If any kind of I/O error occurs.

Description

  This method reads an object from the given stream. This method has full responsibility for restoring the object's state. The implementation of `readExternal()` should read data in the format that is written out by `writeExternal()`. In general, an implementation should call methods of `DataInput` to read primitive types and methods of `ObjectInput` to read objects, strings, and arrays.

### writeExternal

####

     public abstract void writeExternal(ObjectOutput out) throws IOException

Parameters

  `out`

    The object output stream to use.

Throws

  `IOException`

    If any kind of I/O error occurs.

Description

  This method writes an object to the given stream. This method has full responsibility for saving the object's state. The implementation of `writeExternal()` should write data in the format that is read by `readExternal()`. In general, an implementation should call methods of `DataOutput` to write primitive types and methods of `ObjectOutput` to write objects, strings, and arrays.

## See Also

`ClassNotFoundException`, `DataInput`, `DataOutput`, `IOException`, `ObjectInput`, `ObjectOutput`, `Serializable`

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