如何使ObjectInputStream从java文件中读取所有对象？

Question

我有一个存储对象的文件，还有一个*getAll()方法，它需要返回List<Secretary>。但是，我只看到控制台中打印单个对象。我搜索了这个问题，并尝试了3种方法--，但是它没有工作。

在文件中插入对象的插入方法是：

@Override
    public Secretary insert(Secretary t) {
        try { 
            System.out.println("insert called");
        FileOutputStream file = new FileOutputStream 
                                           (filename,true); 
        
            ObjectOutputStream out = new ObjectOutputStream 
                                           (file); 
  
             Method for serialization of object 
            out.writeObject(t); 
  
            out.close(); 
            file.close(); 
  
            return t;       
        } 
  
       catch (IOException ex) { 
            ex.printStackTrace();
        } 
  
        return null;
    }

我已经声明追加模式为真，因为我的文件在保存时被新对象所替换。

因此，我需要从文件中获取所有对象，并将其分配给一个列表。

public class SecretaryDaoImpl implements SecretaryDAO{

    private  String filename = "secretary.txt";
    private Secretary sec=null;
    @Override
        public List<Secretary> getAll() {
    //Method 1  
            
     try { 
     
                Reading the object from a file 
               FileInputStream file = new FileInputStream 
                                            (filename); 
               ObjectInputStream in = new ObjectInputStream 
                                            (file); 
                List<Secretary> secList=new ArrayList<>();
                 Method for deserialization of object 
               secList.add((Secretary)in.readObject()); 
      
               in.close(); 
                file.close(); 
              System.out.println("Object has been deserialized\n"
                                    + "Data after Deserialization."); 
                 
           System.out.println("secList is" +secList);
              return secList;
               
            } 
     
            catch (IOException ex) { 
                System.out.println("Secreatary file not found");
                return null;
            } 
      
            catch (ClassNotFoundException ex) { 
               System.out.println("ClassNotFoundException" + 
                                    " is caught"); 
           } 
             return null;
    
    //Method 2  
              List<Secretary> secList=new ArrayList<>();
            ObjectInputStream objectinputstream = null;
            try {
        FileInputStream streamIn = new FileInputStream(filename);
        objectinputstream = new ObjectInputStream(streamIn);
        List<Secretary> readCase = (List<Secretary>) objectinputstream.readObject();
        for(Secretary s:readCase){
            secList.add(s);
        }
        System.out.println("seclist is" + secList);
        return secList;
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        if(objectinputstream != null){
            try {
                objectinputstream.close();
            } catch (IOException ex) {
                Logger.getLogger(SecretaryDaoImpl.class.getName()).log(Level.SEVERE, null, ex);
            }
        } 
    }
    //Method 3
            try{
             File file = new File(filename);
            List<Secretary> list = new ArrayList<>();
            if (file.exists()) {
                try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file))) {
                    list.add((Secretary) ois.readObject());
                }
            }
                System.out.println("getall is"+list);
            }
            catch(Exception e){
                
            }
            return null;
        }
}

我已经注释掉了我的代码，但是在这里，当我在堆栈溢出中发布时，我取消了对所有代码的注释。我的Secretary.java是：

package com.npsc.entity;

import java.io.Serializable;

/**
 *
 * @author Ashwin
 */
public class Secretary implements Serializable {
     
    private static final long serialVersionUID = 6529685098267757690L;
    private int id;
    private String userName;
    private String password;
    private Branch branch;

    public String getUserName() {
        return userName;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    
    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Secretary(String userName, String password) {
        this.userName = userName;
        this.password = password;
        
    }

    public Branch getBranch() {
        return branch;
    }

    public void setBranch(Branch branch) {
        this.branch = branch;
    }

    @Override
    public String toString() {
        return "Secretary{" + "id=" + id + ", userName=" + userName + ", password=" + password + ", branch=" + branch + '}';
    }
    
    
    
}

在执行插入操作时，我的txt文件保存对象是：

​

但是，我无法读取所有的对象并添加我所面临的问题的list.Where？

Answer 1

您将需要存储在文件中，将秘书对象的数量读取回去。然后，您可以确定要读取多少个实体，从而重新填充列表。

类似于：

List<Secretary> list;
private void persistList(ObjectOutputStream out) {
   out.writeInt(list.size());
   for (Secretary sec : list) {
      out.writeObject(sec);
   }
}

然后读到：

private List<Secretary> readFromStream(ObjectInputStream in) {
   int numObjects = in.readInt();
   List<Secretary> result = new ArrayList<>(numObjects);
   for (int i = 0; i < numObjects; i++) {
      result.add((Secretary)in.readObject());
   }
   return result;
}

这只是技术的一个草图(忽略错误处理、流打开/关闭等)；主要是集成记录列表大小的概念，然后将许多秘书读入到您现有的代码中。

Answer 2

编写一个List<Secretary>文件并读取相同的文件，然后您将拥有所有。

write (Secretary s) {
 read List<Secretary> currentList ;
 currentList.add(s)
 write List<Secretary> 
}