JAVA基础之序列化

今天来讲讲序列化以及反序列化

Serializable接口，没有method，实现该接口表明可以序列化以及反序列化
ObjectOutputStream，将Object序列化为stream流
ObjectInputStream，通过stream流反序列化转化为Object
序列化时，计算出一个Serial version unique identifier标识这个class（安全的哈希值），并将这个哈希值其存储到stream中，而反序列化时java再次计算这个class的标识值（如果class类的定义已改变则值也会发生变化），新计算的值与从stream取出来的值做比较，如果不一致，表示该类的序列化版本与我们拥有的当前类定义不兼容。会产生InvalidClassException。

public class BankAccount implements Serializable {
    private String id;
    private int balance = 0;
    public BankAccount(String id){
        this.id = id;
    }
    public BankAccount(String id, int startBalance){
        this.id = id;
        balance = startBalance;
    }
    public String getId(){
        return id;
    }
    public synchronized int getBalance(){
        return balance;
    }
    public synchronized void deposit(int amount){
        balance += amount;
    }
    public synchronized void withdrawal(int amount){
        balance -= amount;
    }
}

序列化与反序列化方法

public class Main {
    public static void main(String[] args) {
        BankAccount ba = new BankAccount("10",500);
        ba.deposit(250);
        saveAccount(ba,"/Users/buxuesong/Documents/svn_code/demo/account.dat");
        BankAccount bb = loadAccount("/Users/buxuesong/Documents/svn_code/demo/account.dat");
        System.out.println(bb.getId() +" | " + bb.getBalance());
    }
    private static void saveAccount(BankAccount ba, String fileName){
        try(ObjectOutputStream os = new ObjectOutputStream(Files.newOutputStream(Paths.get(fileName)))){
            os.writeObject(ba);
        }catch(Exception e){
            System.out.println(e.getSuppressed() +" | "+ e.getMessage());
        }
    }
    private static BankAccount loadAccount(String fileName){
        BankAccount ba = null;
        try(ObjectInputStream oi = new ObjectInputStream(Files.newInputStream(Paths.get(fileName)))){
            ba = (BankAccount) oi.readObject();
        }catch(Exception e){
            System.out.println(e.getSuppressed() +" | "+ e.getMessage());
        }
        return ba;
    }
}
输出
10 | 750

当改变了class的定义字段后，与原有序列化的唯一标识不同，导致无法反序列化，这就需要自定义serialVersionUID，以后修改了该类，也会可以反序列化回来，只是新增的字段默认值为空或者是原始默认值（int的话就是0）

private static final long serialVersionUID = -23324324324L;

还可以自定义序列化以及反序列化的方法，在BankAccount中

private void writeObject(ObjectOutputStream out) throws IOException {
    out.defaultWriteObject();
}

private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException{
    ObjectInputStream.GetField fields = in.readFields();
    id = (String) fields.get("id",null);
    balance = fields.get("balance",0);
    lastTxType = fields.get("lastTxType",'u');
    lastTxAmount = fields.get("lastTxAmount",-1);
}

BankAccount bb = loadAccount("/Users/buxuesong/Documents/svn_code/demo/account.dat");
System.out.println(bb.getId() +" | " + bb.getBalance()+" | " +bb.getLastTxType()+" | "+bb.getLastTxAmount());
输出
10 | 750 | u | -1

还可以针对某些字段不必须做序列化处理，需要通过transient修饰该字段，可以通过其它方式获取回来，这样节省了序列化的时间
具体如下

public class AccountGroup implements Serializable {
    private static final long serialVersionUID = 106962907155393149L;

    private Map<String, BankAccount> accountMap = new HashMap();
    private transient int totalBalance;
    public int getTotalBalance(){
        return totalBalance;
    }
    public void addAccount(BankAccount account){
        totalBalance += account.getBalance();
        accountMap.put(account.getId(), account);
    }
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        for(BankAccount account: accountMap.values())
            totalBalance += account.getBalance();
    }
}

执行方法中的存储到流以及从流中取回的方法

private static void saveGroup(AccountGroup g, String fileName){
    try(ObjectOutputStream os = new ObjectOutputStream(Files.newOutputStream(Paths.get(fileName)))){
        os.writeObject(g);
    }catch(Exception e){
        System.out.println(e.getSuppressed() +" | "+ e.getMessage());
    }
}

private static AccountGroup loadGroup(String fileName){
    AccountGroup g = null;
    try(ObjectInputStream oi = new ObjectInputStream(Files.newInputStream(Paths.get(fileName)))){
        g = (AccountGroup) oi.readObject();
    }catch(Exception e){
        System.out.println(e.getSuppressed() +" | "+ e.getMessage());
    }
    return g;
}

执行方法：

BankAccount acct1 = new BankAccount("1234", 500);
BankAccount acct2 = new BankAccount("9866", 750);
AccountGroup group = new AccountGroup();
group.addAccount(acct1);
group.addAccount(acct2);
saveGroup(group, "/Users/buxuesong/Documents/svn_code/demo/group.dat");
AccountGroup group2 = loadGroup("/Users/buxuesong/Documents/svn_code/demo/group.dat");
System.out.println("group2.getTotalBalance:"+group2.getTotalBalance());

group2.getTotalBalance:1250

其他序列化方法，实现Externalizable接口

void writeExternal(ObjectOutput out);
void readExternal(ObjectInput in);