JVM (Java virtual machine), their internal state can only be maintained in memory. Once When the JVM stops working, the internal state disappears, so it is non-persistent.
Persistence), the usual approach is to save it to a file or database.
serialize. Through serialization, you can easily serialize it in the JVM. The active object is converted into a byte array (stream) for storage.
class Person implements Serializable { private String name; private int age; public Person(String name ,int age){ this.name = name; this.age =age; } public String getName() { return name; } public int getAge() { return age; } @Override public String toString() { return "name is " + name + " , age is " + age; } }public class Test { private final static String TEMPFILE = "E:" + File.separator + "test.txt"; public static void main(String[] args) { Person person = new Person("test",100); write(person); // 关键 -> 序列化之后重新对对象进行赋值 person = new Person("hello",999); read(person); // 输出结果: // name is test , age is 100,序列化成功 // 反序列化成功,name is test , age is 100 } // 通过 ObjectOutputStream 进行对象的序列化操作 private static void write(Person person) { try { ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(TEMPFILE)); oos.writeObject(person); oos.close(); System.out.println(person+",序列化成功"); } catch (Exception e) { e.printStackTrace(); } } // 通过 ObjectInputStream 进行对象的反序列化操作 private static void read(Person person) { try { ObjectInputStream ois = new ObjectInputStream(new FileInputStream(TEMPFILE)); person = (Person) ois.readObject(); ois.close(); System.out.println("反序列化成功.,"+person); } catch (Exception e) { e.printStackTrace(); } } }
is not unserializable).
// 序列化过程、调用过程与上述例子一致,省略代码... // 这里只对内部类 Person 的 age 属性进行修改class Person implements Serializable { // 用 transient 修该变量 private transient int age; // 省略部分代码...} // 调用后的输出结果: // name is test , age is 100,序列化成功 // 反序列化成功.,name is test , age is 0
It is not that they cannot be serialized).
// 序列化过程、调用过程与上述例子一致,省略代码... // 这里只对内部类 Person 的 name 属性进行修改class Person implements Serializable { // 用 static 修该变量 private static String name; // 省略部分代码...} // 输出结果: // name is test , age is 100,序列化成功//反序列化成功.,name is hello , age is 100
cannot be serialized.
// 序列化过程、调用过程与上述例子一致,省略代码... // 这里只对内部类 Person 的 name 属性进行修改class Person implements Serializable { //新增成员变量 private Thread myThread = new Thread(); //省略部分代码...} // 输出结果(抛出异常): // Caused by: java.io.NotSerializableException: java.lang.Thread
// 序列化操作代码与上面一致... // 这里只对内部类 Person 的属性进行修改。class Person implements Serializable { private static String name; private transient int age; // 自定义序列化操作 private void writeObject(ObjectOutputStream out) throws IOException{ out.defaultWriteObject(); out.writeObject(name); out.writeInt(age); } // 自定义反序列化操作 private void readObject(ObjectInputStream in) throws IOException,ClassNotFoundException{ in.defaultReadObject(); name = (String) in.readObject(); age = in.readInt(); } // 省略部分代码...} // 输出结果: // name is test , age is 100,序列化成功 // 反序列化成功,name is test , age is 100
writeExternal and readExternal methods for custom serialization and deserialization ization process.
parameterless constructor.
// 序列化操作代码与上面一致,这里只对内部类 Person 的进行修改。 class Person implements Externalizable { private static String name; private transient int age; // 重点 ->必须有无参构造函数 public Person(){ } public Person(String name ,int age){ this.name = name; this.age =age; } public String getName() { return name; } public int getAge() { return age; } @Override public String toString() { return "name is " + name + " , age is " + age; } // 实现接口的方法 @Override public void writeExternal(ObjectOutput out) throws IOException { out.writeObject(name); out.writeInt(age); } // 实现接口的方法 @Override public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { name = (String) in.readObject(); age = in.readInt(); } } // 输出结果: // name is test , age is 100,序列化成功 // 反序列化成功,name is test , age is 100
method is modified, the serialVersionUID ** remains unchanged **.
properties are modified, the regenerated serialVersionUID** will change**.
因此说明序列化是作用于对象属性上的。
下面通过实例来探究下 serialVersionUID 的具体作用:
首先我们对对象进行序列化操作(这里取消了反序列化的操作)
// 序列化操作代码与上面例子一致...// 这里取消了反序列化的操作public static void main(String[] args) { Person person = new Person("test",100); write(person); person = new Person("java", 200); //read(person);}
然后新增一个对象的属性,再进行反序列化操作
// 省略部分代码,与上面的代码一致...class Person implements Serializable { private String name; private age; //新增成员变量 private int phone; //省略部分代码... } }public static void main(String[] args) { Person person = new Person("test",100); //write(person); person = new Person("java", 200); read(person); }// 输出结果(抛出异常):// java.io.InvalidClassException: Person;
观察代码,在序列化对象并没有添加 serialVersionUID 的情况,在对象序列化之后如果改变了对象的属性,反序列化就会抛出异常。如果对象添加了 serialVersionUID 就不会出现这种情况,这里就不验证了。
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