Java IO系列（五）：读写对象ObjectOutputStream和ObjectInputStream

转载请注明出处：http://blog.csdn.net/jeffleo/article/details/52266577

有必要多看几遍的

关于字符和字节，例如文本文件，XML这些都是用字符流来读取和写入。而如RAR，EXE文件，图片等非文本，则用字节流来读取和写入。

读写对象，传输对象在Java中很常使用，在javaBean中就经常用到。一般来说，对象要能被对写，自身要实现特定的接口，Serializable或Externalizable接口。

实现Serializable接口对象的读写

实现了Serializable的对象，会被全序列化，当我们在进行序列化时，一般对象中存在我们不需要序列化的数据。但Serializable会全序列化，影响效率，一般来说，可能并不是最好的选择。

class Person implements Serializable{
private int age;
private String name;

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

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

public class TestObject {
public static void main(String[] args) throws IOException, ClassNotFoundException {
String path = "F:/io/person.txt";
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(path));
out.writeObject(new Person(20, "nick"));
out.writeObject(new Person(21, "liu"));
out.writeObject(new Person(22, "mike"));
out.close();

ObjectInputStream in = new ObjectInputStream(new FileInputStream(path));
for (int i = 0; i < 3; i++) {
System.out.println(in.readObject());
}
in.close();
}
}

结果为：

[age:20,name:nick]
[age:21,name:liu]
[age:22,name:mike]

transient使实现Serializable对象不全序列化

transient是一个关键字，配合Serializable使用，表示该数据不需要序列化。

class Person implements Serializable{
private int age;
private transient String name;//注意这里使用了transient

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

@Override
public String toString(){
return "[age:" + age + ",name:" + name + "]";
}
}
public class TestObject {
public static void main(String[] args) throws IOException, ClassNotFoundException {
String path = "F:/io/person.txt";
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(path));
out.writeObject(new Person(20, "nick"));
out.writeObject(new Person(21, "liu"));
out.writeObject(new Person(22, "mike"));
out.close();

ObjectInputStream in = new ObjectInputStream(new FileInputStream(path));
for (int i = 0; i < 3; i++) {
System.out.println(in.readObject());
}
in.close();
}
}

结果为：

[age:20,name:null]
[age:21,name:null]
[age:22,name:null]

可见，name确实没有被序列化

实现Externalizable接口对象的局部数据读写

实现Externalizable，则必须实现writeExternal(ObjectOutput out)和readExternal(ObjectInput in)这两个方法，而这两个方法正是关键。

对于Serializable对象，对象不会调用它的构造器。但是Externalizable对象会调用它所有的默认构造器。而我们一般对对象初始化，不是在默认的构造器，这样的话，对象中的数据都没有初始化。

所以我们要在writeExternal中，写我们需要的数据，在readExternal中取我们需要的数据。

class Person implements Externalizable{
private int age;
private String name;
private String address;

public Person(){

}

public Person(int age, String name, String address){
this.age = age;
this.name = name;
this.address = address;
}

@Override
public void writeExternal(ObjectOutput out) throws IOException {
out.writeInt(age);
out.writeObject(name);
}

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

@Override
public String toString(){
return "[age:" + age + ",name:" + name + ",address:" + address + "]";
}
}
public class TestObject {
public static void main(String[] args) throws IOException, ClassNotFoundException {
String path = "F:/io/person.txt";
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(path));
out.writeObject(new Person(20, "nick", "beijing"));
out.writeObject(new Person(21, "liu", "guangzhou"));
out.writeObject(new Person(22, "mike", "shanghai"));
out.close();

ObjectInputStream in = new ObjectInputStream(new FileInputStream(path));
for (int i = 0; i < 3; i++) {
System.out.println((Person) in.readObject());
}
in.close();
}
}

结果为：

[age:20,name:nick,address:null]
[age:21,name:liu,address:null]
[age:22,name:mike,address:null]

可见，在writeExternal中，并没有写入address，因为我不需要它。所以我只读取到age和name。