有必要多看幾遍的

關于字符和字節，例如文本文件，XML這些都是用字符流來讀取和寫入。而如RAR，EXE文件，圖片等非文本，則用字節流來讀取和寫入。

讀寫對象，傳輸對象在Java中很常使用，在javaBean中就經常用到。一般來說，對象要能被對寫，自身要實現特定的接口，Serializable或Externalizable接口。

實現Serializable接口對象的讀寫

實現了Serializable的對象，會被全序列化，當我們在進行序列化時，一般對象中存在我們不需要序列化的數據。但Serializable會全序列化，影響效率，一般來說，可能并不是最好的選擇。

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();

}

}

結果為：

[age:20,name:nick]

[age:21,name:liu]

[age:22,name:mike]

transient使實現Serializable對象不全序列化

transient是一個關鍵字，配合Serializable使用，表示該數據不需要序列化。

class Person implements Serializable{

private int age;

private transient String name;//注意這里使用了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();

}

}

結果為：

[age:20,name:null]

[age:21,name:null]

[age:22,name:null]

可見，name確實沒有被序列化

實現Externalizable接口對象的局部數據讀寫

實現Externalizable，則必須實現writeExternal(ObjectOutput out)和readExternal(ObjectInput in)這兩個方法，而這兩個方法正是關鍵。

對于Serializable對象，對象不會調用它的構造器。但是Externalizable對象會調用它所有的默認構造器。而我們一般對對象初始化，不是在默認的構造器，這樣的話，對象中的數據都沒有初始化。

所以我們要在writeExternal中，寫我們需要的數據，在readExternal中取我們需要的數據。

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();

}

}

結果為：

[age:20,name:nick,address:null]

[age:21,name:liu,address:null]

[age:22,name:mike,address:null]

可見，在writeExternal中，并沒有寫入address，因為我不需要它。所以我只讀取到age和name。

總結

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