分析輪子（四）- 我也玩一把 Serializable.java

前言：在寫 分析輪子（一）-ArrayList.java 的時候曾經下過一個結論 “實現Serializable介面，表示ArrayList是可序列化的”，這個結論是以往學習的經驗所得，並且平時在程式設計的時候也遇到過其他的問題，比如：在寫 IDEA使用筆記（八）——自動生成 serialVersionUID 的設定 的時候，其實就遇到了一個物件序列化和反序列化相關的問題，後來解決了，不過沒有深入下去和總結一下。程式設計這件事情，最好實驗一把，就算是他人已經研究明白的東西，自己如果不動手試試，可能印象總不是特別的深刻，哪怕隨便玩一下，也許都會有完全不同的收穫。

 

注：玩的是JDK1.7版 紙上得來終覺淺，絕知此事要躬行。（自勉之。。。）

 

一：如果不實現 Serializable.java 介面，我要序列化物件會怎樣呢？

1）來個簡單的Bean，故意不實現 Serializable.java介面，如下所示

/**
 * @description：人類
 * @author：godtrue
 * @create：2018-09-09
 */
public class Person {//注意這裡哈！
    /**
     * 身份證號
     */
    private int id;

    /**
     * 姓名
     */
    private String name;

    /**
     * 性別
     */
    private boolean sex;

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isSex() {
        return sex;
    }

    public void setSex(boolean sex) {
        this.sex = sex;
    }

    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder("Person{");
        sb.append("id=").append(id);
        sb.append(", name='").append(name).append('\'');
        sb.append(", sex=").append(sex);
        sb.append('}');
        return sb.toString();
    }
}

2）來個序列化和反序列化的測試類，跑一下，看看情況如何？

/**
 * @description：序列化和反序列化測試類
 * @author：godtrue
 * @create：2018-09-09
 */
public class SerializeAndDeserialize{
    /**
    *
    *@description: 測試入口，主方法
    *@param args
    *@return: void
    *@author: godtrue
    *@createTime: 2018-09-09
    *@version: v1.0
    */
    public static void main(String[] args){
        Person person = new Person();
        person.setId(1111);
        person.setName("雙十一");
        person.setSex(true);

        try {
            serializePerson(person);
            Person personTemp = deserializePerson();
            System.out.println(personTemp);
        }catch (Exception e){
            e.printStackTrace();
        }

    }

    /**
    *
    *@description: 序列化人類物件方法
    *@param person
    *@return: void
    *@author: godtrue
    *@createTime: 2018-09-09
    *@version: v1.0
    */
    private static void serializePerson(Person person) throws IOException{
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("D://PersonInfo.text"));
        objectOutputStream.writeObject(person);
        System.out.println("serialize person success");
        objectOutputStream.close();
    }

    /**
    *
    *@description: 反序列化人類方法
    *@param
    *@return: com.godtrue.Person
    *@author: gotrue
    *@createTime: 2018-09-09
    *@version: v1.0
    */
    private static Person deserializePerson() throws ClassNotFoundException,IOException{
        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("D://PersonInfo.text"));
        Person person = (Person) objectInputStream.readObject();
        System.out.println("deserialize person success");
        return person;
    }
}

3）不實現 Serializable.java 介面，進行物件序列化的後果很嚴重（序列化不成），並且丟擲 java.io.NotSerializableException 異常

Connected to the target VM, address: '127.0.0.1:53429', transport: 'socket'
java.io.NotSerializableException: com.godtrue.Person
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)
    at com.godtrue.SerializeAndDeserialize.serializePerson(SerializeAndDeserialize.java:28)
    at com.godtrue.SerializeAndDeserialize.main(SerializeAndDeserialize.java:18)
Disconnected from the target VM, address: '127.0.0.1:53429', transport: 'socket'

Process finished with exit code 0

4）跟一下，看看這個異常時哪裡丟擲來的

    /**
     * Write the specified object to the ObjectOutputStream.  The class of the
     * object, the signature of the class, and the values of the non-transient
     * and non-static fields of the class and all of its supertypes are
     * written.  Default serialization for a class can be overridden using the
     * writeObject and the readObject methods.  Objects referenced by this
     * object are written transitively so that a complete equivalent graph of
     * objects can be reconstructed by an ObjectInputStream.
     *
     * <p>Exceptions are thrown for problems with the OutputStream and for
     * classes that should not be serialized.  All exceptions are fatal to the
     * OutputStream, which is left in an indeterminate state, and it is up to
     * the caller to ignore or recover the stream state.
     *
     * @throws  InvalidClassException Something is wrong with a class used by
     *          serialization.
     * @throws  NotSerializableException Some object to be serialized does not
     *          implement the java.io.Serializable interface.
     * @throws  IOException Any exception thrown by the underlying
     *          OutputStream.
     */
    public final void writeObject(Object obj) throws IOException {
        if (enableOverride) {
            writeObjectOverride(obj);
            return;
        }
        try {
            writeObject0(obj, false);//往這裡走啦！
        } catch (IOException ex) {
            if (depth == 0) {
                writeFatalException(ex);
            }
            throw ex;
        }
    }

/**
     * Underlying writeObject/writeUnshared implementation.
     */
    private void writeObject0(Object obj, boolean unshared)
        throws IOException
    {
        boolean oldMode = bout.setBlockDataMode(false);
        depth++;
        try {
            // handle previously written and non-replaceable objects
            int h;
            if ((obj = subs.lookup(obj)) == null) {
                writeNull();
                return;
            } else if (!unshared && (h = handles.lookup(obj)) != -1) {
                writeHandle(h);
                return;
            } else if (obj instanceof Class) {
                writeClass((Class) obj, unshared);
                return;
            } else if (obj instanceof ObjectStreamClass) {
                writeClassDesc((ObjectStreamClass) obj, unshared);
                return;
            }

            // check for replacement object
            Object orig = obj;
            Class cl = obj.getClass();
            ObjectStreamClass desc;
            for (;;) {
                // REMIND: skip this check for strings/arrays?
                Class repCl;
                desc = ObjectStreamClass.lookup(cl, true);
                if (!desc.hasWriteReplaceMethod() ||
                    (obj = desc.invokeWriteReplace(obj)) == null ||
                    (repCl = obj.getClass()) == cl)
                {
                    break;
                }
                cl = repCl;
            }
            if (enableReplace) {
                Object rep = replaceObject(obj);
                if (rep != obj && rep != null) {
                    cl = rep.getClass();
                    desc = ObjectStreamClass.lookup(cl, true);
                }
                obj = rep;
            }

            // if object replaced, run through original checks a second time
            if (obj != orig) {
                subs.assign(orig, obj);
                if (obj == null) {
                    writeNull();
                    return;
                } else if (!unshared && (h = handles.lookup(obj)) != -1) {
                    writeHandle(h);
                    return;
                } else if (obj instanceof Class) {
                    writeClass((Class) obj, unshared);
                    return;
                } else if (obj instanceof ObjectStreamClass) {
                    writeClassDesc((ObjectStreamClass) obj, unshared);
                    return;
                }
            }

            // remaining cases
            if (obj instanceof String) {
                writeString((String) obj, unshared);
            } else if (cl.isArray()) {
                writeArray(obj, desc, unshared);
            } else if (obj instanceof Enum) {
                writeEnum((Enum) obj, desc, unshared);
            } else if (obj instanceof Serializable) {
                writeOrdinaryObject(obj, desc, unshared);
            } else {
                if (extendedDebugInfo) {
                    throw new NotSerializableException(
                        cl.getName() + "\n" + debugInfoStack.toString());
                } else {
                    throw new NotSerializableException(cl.getName());//最終在這裡丟擲了對應的異常
                }
            }
        } finally {
            depth--;
            bout.setBlockDataMode(oldMode);
        }
    }

二：嗯，那好吧！現在終於明白如果不實現 Serializable.java 介面，序列化物件的後果了，那就實現一下吧！畢竟JAVA平臺都這麼規定了，除非換個平臺了！

1）調整 Person.java 類，使其實現 Serializable.java 介面，如下所示

/**
 * @description：人類
 * @author：godtrue
 * @create：2018-09-09
 */
public class Person implements Serializable{//注意這裡啦！
    /**
     * 身份證號
     */
    private int id;

    /**
     * 姓名
     */
    private String name;

    /**
     * 性別
     */
    private boolean sex;

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isSex() {
        return sex;
    }

    public void setSex(boolean sex) {
        this.sex = sex;
    }

    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder("Person{");
        sb.append("id=").append(id);
        sb.append(", name='").append(name).append('\'');
        sb.append(", sex=").append(sex);
        sb.append('}');
        return sb.toString();
    }
}

2）序列化和反序列化的測試類 原封未動，這裡就不貼出來了

3）Person.java 實現 Serializable.java 後，物件的序列化和反序列化都成功了，如下所示

serialize person success
deserialize person success
Person{id=1111, name='雙十一', sex=true}

Process finished with exit code 0

4）序列化到檔案中的資訊，如下所示

 

三：serialVersionUID 的作用

實驗步驟：

1）先將 Person.java 類的物件序列化到 D://PersonInfo.text 檔案中（在Person.java類中沒有顯示宣告 serialVersionUID）

2）為Person.java 類新增一個年齡屬性

    /**
     * 年齡
     */
    private int age;

3）反序列化 D://PersonInfo.text 檔案中物件資訊

4）執行結果如下（丟擲了 java.io.InvalidClassException ）：

Connected to the target VM, address: '127.0.0.1:51721', transport: 'socket'
Disconnected from the target VM, address: '127.0.0.1:51721', transport: 'socket'
java.io.InvalidClassException: com.godtrue.Person; local class incompatible: stream classdesc serialVersionUID = 2168487965208983906, local class serialVersionUID = 7553450974679663255
    at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:617)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1622)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1517)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1771)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1350)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:370)
    at com.godtrue.SerializeAndDeserialize.deserializePerson(SerializeAndDeserialize.java:63)
    at com.godtrue.SerializeAndDeserialize.main(SerializeAndDeserialize.java:28)

Process finished with exit code 0

5）分析——看拋錯的日誌資訊，引起這個問題的原因是因為，序列化和反序列化資訊中的 serialVersionUID 這個屬性的值不一致造成的，那問題來了 serialVersionUID 這個屬性哪裡來的？這個屬性的值又是哪裡來的？為什麼同一個物件的序列化和反序列化資訊中的有些資訊不一樣呢？

5-1）serialVersionUID 這個屬性是JDK工具新增上去
5-2）serialVersionUID 這個屬性的值，如果沒有顯式指明，則會由JDK工具自動生成，如果顯式宣告瞭，則使用顯式宣告的
5-3）serialVersionUID 這個屬性的預設生成規則，和類中的資訊有關，如果類有所改變，則會影響此屬性的值的生成

6）繼續試驗下，5）中的結論，試驗步驟如下：
6-1）先將Person.java中的 age 屬性去掉，然後顯式的宣告 private static final long serialVersionUID=1L;
6-2）然後，將 Person.java 類對應的物件資訊，序列化到 D://PersonInfo.text 檔案中
6-3）然後，將 Person.java 類的 age 屬性再新增回去
6-4）然後，將Person.java 類的物件資訊反序列化為物件，
6-5）試驗ok了，序列化和反序列化都沒有問題，只是反序列化後的物件資訊中 age 屬性的值是預設屬性 0 

7）繼續試驗，逆向的驗證一下反序列化，試驗步驟如下：

7-1）先將Person.java中的 age 屬性新增上，然後顯式的宣告 private static final long serialVersionUID=1L;
7-2）然後，將 Person.java 類對應的物件資訊，序列化到 D://PersonInfo.text 檔案中
7-3）然後，將 Person.java 類的 age 屬性再去掉
7-4）然後，將Person.java 類的物件資訊反序列化為物件，
7-5）試驗ok了，序列化和反序列化都沒有問題，只是，反序列化後的物件資訊中沒有 age 屬性而已，（去掉後，反序列化當然是沒有的）

8）從上述的試驗中，我們可以發現一些有趣的事情
8-1）當 serialVersionUID 屬性的值，不一致時，即使是同一個類，他的序列化和反序列化前後的屬性沒有增減，也是不能正確反序列化的
8-2）當 serialVersionUID 屬性的值，一致時，同一個類，他的序列化和反序列化前後的屬性有增減，也能正確反序列化的

參考：
http://www.oracle.com/technetwork/articles/java/javaserial-1536170.html
https://www.ibm.com/developerworks/cn/java/j-lo-serial/index.html
https://www.cnblogs.com/xdp-gacl/p/3777987.html
http://ju.outofmemory.cn/entry/298220
https://blog.csdn.net/qq_27093465/article/details/78544505
https://blog.csdn.net/jason_279/article/details/52947093
https://blog.csdn.net/u011784767/article/details/78156319?locationNum=2&fps=1
https://www.cnblogs.com/wangg-mail/p/4354709.html
https://blog.csdn.net/leixingbang1989/article/details/50556966
https://www.cnblogs.com/DSNFZ/articles/7618470.html
http://www.cnblogs.com/huhx/p/serializable.html
https://blog.csdn.net/so_geili/article/details/78931742
https://www.oschina.net/question/4873_23270
https://www.cnblogs.com/qq3111901846/p/7894532.html
https://www.cnblogs.com/gtaxmjld/p/4866931.html
https://blog.csdn.net/zhangliao613/article/details/51086562