Java中与Serializable接口相关的一些方法执行顺序

绝地灬酷狼 2021-09-21 11:06 185阅读 0赞

## 自定义 writeObject 方法和 readObject 方法 ##

如果在实现Serializable接口的类中自定义 writeObject 方法和 readObject 方法，则不会调用默认的方法：

package serializable;
    
    import java.io.FileInputStream;
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.io.Serializable;
    
    
    public class Student implements Serializable {
        private static final long serialVersionUID = 140932343675039705L;
    
        private String name;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Override
        public String toString(){
            return "学生:" + name;
        }
    
        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            System.out.println("readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            System.out.println("writeObject exec...");
        }
    
        public static void main(String[] args) throws IOException, ClassNotFoundException {
            Student student = new Student();
            student.setName("小明");
            String filePath = "/Users/dcc/Downloads/student";
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(filePath));
            out.writeObject(student);
            out.close();
    
            ObjectInputStream in = new ObjectInputStream(new FileInputStream(filePath));
            Student student1 = (Student) in.readObject();
            System.out.println(student1);
        }
    
    }

程序输出结果：

writeObject exec...
    readObject exec...
    学生:null

因为在 writeObject 方法中只打印了日志，没有序列化 name 字段，所以 readObject 方法没有反序列化出 name.可以在自定义的writeObject 方法和 readObject 方法中调用默认方法：

private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            System.out.println("readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            out.defaultWriteObject();
            System.out.println("writeObject exec...");
        }

修改之后 name 字段序列化和反序列化均成功，程序输出：

writeObject exec...
    readObject exec...
    学生:小明

## writeReplace 方法和 readResolve 方法 ##

writeReplace 方法可以替换序列化的对象；readResolve 方法是替换反序列化的对象：

看如下代码：

package serializable;
    
    import java.io.FileInputStream;
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.io.ObjectStreamException;
    import java.io.Serializable;
    
    
    public class Student implements Serializable {
        private static final long serialVersionUID = 140932343675039705L;
    
        private String name;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Override
        public String toString(){
            return "学生:" + name;
        }
    
        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            System.out.println("readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            out.defaultWriteObject();
            System.out.println("writeObject exec...");
        }
    
        private Object writeReplace() throws ObjectStreamException{
            System.out.println("writeReplace exec...");
            return this;
        }
    
        private Object readResolve() throws ObjectStreamException{
            System.out.println("readResolve exec...");
            return this;
        }
    
        public static void main(String[] args) throws IOException, ClassNotFoundException {
            Student student = new Student();
            student.setName("小明");
            String filePath = "/Users/dcc/Downloads/student";
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(filePath));
            out.writeObject(student);
            out.close();
    
            ObjectInputStream in = new ObjectInputStream(new FileInputStream(filePath));
            Student teacher = (Student) in.readObject();
            System.out.println(teacher);
        }
    
    }

程序运行结果：

writeReplace exec...
    writeObject exec...
    readObject exec...
    readResolve exec...
    学生:小明

可以看出：**writeReplace 方法是在 writeObject 方法之前执行；readResolve 方法是在 readResolve 方法之后执行。**

修改上面的代码：

package serializable;
    
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.io.Serializable;
    
    
    public class Teacher implements Serializable {
        private static final long serialVersionUID = 4644753740588166024L;
    
        private String teacherNo;
    
        public Teacher(String teacherNo) {
            this.teacherNo = teacherNo;
        }
    
        @Override
        public String toString() {
            return "老师:" + teacherNo;
        }
    
        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            System.out.println("Teacher readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            out.defaultWriteObject();
            System.out.println("Teacher writeObject exec...");
        }
    }

public class Student implements Serializable {
        private static final long serialVersionUID = 140932343675039705L;
    
        private String name;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Override
        public String toString(){
            return "学生:" + name;
        }
    
        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            System.out.println("readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            out.defaultWriteObject();
            System.out.println("writeObject exec...");
        }
    
        private Object writeReplace() throws ObjectStreamException{
            System.out.println("writeReplace exec...");
            return new Teacher("007");
        }
    
        public static void main(String[] args) throws IOException, ClassNotFoundException {
            Student student = new Student();
            student.setName("小明");
            String filePath = "/Users/dcc/Downloads/student";
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(filePath));
            out.writeObject(student);
            out.close();
    
            ObjectInputStream in = new ObjectInputStream(new FileInputStream(filePath));
            Student student1 = (Student) in.readObject();
            System.out.println(student1);
        }
    
    }

程序输出：

writeReplace exec...
    Teacher writeObject exec...
    Teacher readObject exec...
    Exception in thread "main" java.lang.ClassCastException: serializable.Teacher cannot be cast to serializable.Student
    	at serializable.Student.main(Student.java:57)

可以看到序列化的小明同学Student对象已经被writeReplace方法替换成了Teacher对象，而且执行的是Teacher类中定义的 writeObject 方法和 readObject 方法。

反序列化处代码修改：

ObjectInputStream in = new ObjectInputStream(new FileInputStream(filePath));
            Teacher teacher = (Teacher) in.readObject();
            System.out.println(teacher);

程序输出结果：

writeReplace exec...
    Teacher writeObject exec...
    Teacher readObject exec...
    老师:007

我们在Student 类中增加 readResolve 方法：

private Object readResolve() throws ObjectStreamException{
            System.out.println("readResolve exec...");
            return new Teacher("001");
        }

此时反序列化得到的老师对象仍是“老师:007”而不是“老师:001”，因为反序列化出的是Teacher对象，不会调用Student的readResolve 方法。

我们接着在Teacher类中添加 readResolve 方法：

private Object readResolve() throws ObjectStreamException{
            System.out.println("Teacher readResolve exec...");
            return new Teacher("001");
        }

此时序列化得到的老师对象才是“老师:001”。

我们把Student和Teacher类中的writeObject 方法、 readObject 方法、writeReplace 方法和、readResolve 方法四种方法都补齐，看一下调用顺序：

package serializable;
    
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.io.ObjectStreamException;
    import java.io.Serializable;
    
    
    public class Teacher implements Serializable {
        private static final long serialVersionUID = 4644753740588166024L;
    
        private String teacherNo;
    
        public Teacher(String teacherNo) {
            this.teacherNo = teacherNo;
        }
    
        @Override
        public String toString() {
            return "老师:" + teacherNo;
        }
    
        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            System.out.println("Teacher readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            out.defaultWriteObject();
            System.out.println("Teacher writeObject exec...");
        }
    
        private Object writeReplace() throws ObjectStreamException {
            System.out.println("Teacher writeReplace exec...");
            return this;
        }
    
        private Object readResolve() throws ObjectStreamException{
            System.out.println("Teacher readResolve exec...");
            return new Teacher("001");
        }
    }

package serializable;
    
    import java.io.FileInputStream;
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.io.ObjectStreamException;
    import java.io.Serializable;
    
    public class Student implements Serializable {
        private static final long serialVersionUID = 140932343675039705L;
    
        private String name;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        @Override
        public String toString(){
            return "学生:" + name;
        }
    
        private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
            in.defaultReadObject();
            System.out.println("readObject exec...");
        }
    
        private void writeObject(ObjectOutputStream out) throws IOException{
            out.defaultWriteObject();
            System.out.println("writeObject exec...");
        }
    
        private Object writeReplace() throws ObjectStreamException{
            System.out.println("writeReplace exec...");
            return new Teacher("007");
        }
    
        private Object readResolve() throws ObjectStreamException{
            System.out.println("readResolve exec...");
            return new Teacher("001");
        }
    
        public static void main(String[] args) throws IOException, ClassNotFoundException {
            Student student = new Student();
            student.setName("小明");
            String filePath = "/Users/dcc/Downloads/student";
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(filePath));
            out.writeObject(student);
            out.close();
    
            ObjectInputStream in = new ObjectInputStream(new FileInputStream(filePath));
            Teacher teacher = (Teacher) in.readObject();
            System.out.println(teacher);
        }
    
    }

程序输出结果：

writeReplace exec...
    Teacher writeReplace exec...
    Teacher writeObject exec...
    Teacher readObject exec...
    Teacher readResolve exec...
    老师:001