java 高效序列化 Externalizable接口

参考：https://www.javacodegeeks.com/2010/07/java-best-practices-high-performance.html

首先需要序列化的类实现Externalizable接口

实现readExternal和writeExternal方法写文件的顺序和读取文件的顺序进行一一对应

需要注意的地方：被序列化的对象需要有一个无参的构造函数。

例子：

被序列化的类：

import java.io.Externalizable;import java.io.IOException;import java.io.ObjectInput;import java.io.ObjectOutput;import java.util.Arrays;import java.util.Date;import java.util.List;public class Employee implements Externalizable {     private String firstName;      private String lastName;      private String socialSecurityNumber;      private String department;      private String position;      private Date hireDate;      private Double salary;      private Employee supervisor;      private List<String> phoneNumbers;      public Employee() {      }      public Employee(String firstName, String lastName,        String socialSecurityNumber, String department, String position,        Date hireDate, Double salary) {       this.firstName = firstName;       this.lastName = lastName;       this.socialSecurityNumber = socialSecurityNumber;       this.department = department;       this.position = position;       this.hireDate = hireDate;       this.salary = salary;      }      public String getFirstName() {       return firstName;      }      public void setFirstName(String firstName) {       this.firstName = firstName;      }      public String getLastName() {       return lastName;      }      public void setLastName(String lastName) {       this.lastName = lastName;      }      public String getSocialSecurityNumber() {       return socialSecurityNumber;      }      public void setSocialSecurityNumber(String socialSecurityNumber) {       this.socialSecurityNumber = socialSecurityNumber;      }      public String getDepartment() {       return department;      }      public void setDepartment(String department) {       this.department = department;      }      public String getPosition() {       return position;      }      public void setPosition(String position) {       this.position = position;      }      public Date getHireDate() {       return hireDate;      }      public void setHireDate(Date hireDate) {       this.hireDate = hireDate;      }      public Double getSalary() {       return salary;      }      public void setSalary(Double salary) {       this.salary = salary;      }      public Employee getSupervisor() {       return supervisor;      }      public void setSupervisor(Employee supervisor) {       this.supervisor = supervisor;      }      public List<String> getPhoneNumbers() {       return phoneNumbers;      }      public void setPhoneNumbers(List<String> phoneNumbers) {       this.phoneNumbers = phoneNumbers;      }      public void readExternal(ObjectInput objectInput) throws IOException,        ClassNotFoundException {       this.firstName = objectInput.readUTF();       this.lastName = objectInput.readUTF();       this.socialSecurityNumber = objectInput.readUTF();       this.department = objectInput.readUTF();       this.position = objectInput.readUTF();       this.hireDate = new Date(objectInput.readLong());       this.salary = objectInput.readDouble();       int attributeCount = objectInput.read();       byte[] attributes = new byte[attributeCount];       objectInput.readFully(attributes);       for (int i = 0; i < attributeCount; i++) {        byte attribute = attributes[i];        switch (attribute) {        case (byte) 0:         this.supervisor = (Employee) objectInput.readObject();         break;        case (byte) 1:         this.phoneNumbers = Arrays.asList(objectInput.readUTF().split(";"));         break;        }       }      }      public void writeExternal(ObjectOutput objectOutput) throws IOException {       objectOutput.writeUTF(firstName);       objectOutput.writeUTF(lastName);       objectOutput.writeUTF(socialSecurityNumber);       objectOutput.writeUTF(department);       objectOutput.writeUTF(position);       objectOutput.writeLong(hireDate.getTime());       objectOutput.writeDouble(salary);       byte[] attributeFlags = new byte[2];       int attributeCount = 0;       if (supervisor != null) {        attributeFlags[0] = (byte) 1;        attributeCount++;       }       if (phoneNumbers != null && !phoneNumbers.isEmpty()) {        attributeFlags[1] = (byte) 1;        attributeCount++;       }       objectOutput.write(attributeCount);       byte[] attributes = new byte[attributeCount];       int j = attributeCount;       for (int i = 0; i < 2; i++)        if (attributeFlags[i] == (byte) 1) {         j--;         attributes[j] = (byte) i;        }       objectOutput.write(attributes);       for (int i = 0; i < attributeCount; i++) {        byte attribute = attributes[i];        switch (attribute) {        case (byte) 0:         objectOutput.writeObject(supervisor);         break;        case (byte) 1:         StringBuilder rowPhoneNumbers = new StringBuilder();         for(int k = 0; k < phoneNumbers.size(); k++)          rowPhoneNumbers.append(phoneNumbers.get(k) + ";");         rowPhoneNumbers.deleteCharAt(rowPhoneNumbers.lastIndexOf(";"));         objectOutput.writeUTF(rowPhoneNumbers.toString());         break;        }       }      }    @Override    public String toString() {        return "Employee [firstName=" + firstName + ", lastName=" + lastName                + ", socialSecurityNumber=" + socialSecurityNumber                + ", department=" + department + ", position=" + position                + ", hireDate=" + hireDate + ", salary=" + salary                + ", supervisor=" + supervisor + ", phoneNumbers="                + phoneNumbers + "]";    }}

序列化方法类

public class Seilize {    public static byte[] serializeObject(Externalizable object, int index)throws Exception {        FileOutputStream fos = null;        ObjectOutputStream oos = null;        byte[] res = null;        try {            fos = new FileOutputStream(new File(index + ".ser"));            oos = new ObjectOutputStream(fos);            object.writeExternal(oos);            oos.flush();            res = object.getClass().getName().getBytes();        } catch (Exception ex) {            throw ex;        } finally {            try {                if (oos != null)                    oos.close();            } catch (Exception e) {                e.printStackTrace();            }        }        return res;    }    public static Externalizable deserializeObject(byte[] rowObject, int index)throws Exception {        FileInputStream fis = null;        ObjectInputStream ois = null;        String objectClassName = null;        Externalizable res = null;        try {            fis = new FileInputStream(new File(index + ".ser"));            objectClassName = new String(rowObject);            ois = new ObjectInputStream(fis);            Class objectClass = Class.forName(objectClassName);            res = (Externalizable) objectClass.newInstance();            res.readExternal(ois);        } catch (Exception ex) {            throw ex;        } finally {            try {                if (ois != null)                    ois.close();            } catch (Exception e) {                e.printStackTrace();            }        }        return res;    }}

测试类

public static void main(String[] args) {        Employee employee = new Employee("ni","hao","ha","helllo","world",new Date(),100.0);         try {            byte[] rowObject = Seilize.serializeObject(employee,1);            System.out.println(rowObject);            System.out.println(Seilize.deserializeObject(rowObject,1));        } catch (Exception e) {            e.printStackTrace();        }    }