Java序列化和反序列化為什么要實(shí)現(xiàn)Serializable接口？
最近公司的在做服務(wù)化, 需要把所有model包里的類都實(shí)現(xiàn)Serializable接口, 同時(shí)還要顯示指定serialVersionUID的值. 聽到這個(gè)需求, 我腦海里就突然出現(xiàn)了好幾個(gè)問題, 比如說:
(1) 序列化和反序列化是什么?
(2) 實(shí)現(xiàn)序列化和反序列化為什么要實(shí)現(xiàn)Serializable接口?
(3) 實(shí)現(xiàn)Serializable接口就算了, 為什么還要顯示指定serialVersionUID的值?
(4) 我要為serialVersionUID指定個(gè)什么值?
下面我們來一一解答這幾個(gè)問題.
序列化和反序列化
序列化：把對象轉(zhuǎn)換為字節(jié)序列的過程稱為對象的序列化.
反序列化：把字節(jié)序列恢復(fù)為對象的過程稱為對象的反序列化.
什么時(shí)候需要用到序列化和反序列化呢?
當(dāng)我們只在本地JVM里運(yùn)行下Java實(shí)例, 這個(gè)時(shí)候是不需要什么序列化和反序列化的, 但當(dāng)我們需要將內(nèi)存中的對象持久化到磁盤, 數(shù)據(jù)庫中時(shí), 當(dāng)我們需要與瀏覽器進(jìn)行交互時(shí), 當(dāng)我們需要實(shí)現(xiàn)RPC時(shí), 這個(gè)時(shí)候就需要序列化和反序列化了.
前兩個(gè)需要用到序列化和反序列化的場景, 是不是讓我們有一個(gè)很大的疑問? 我們在與瀏覽器交互時(shí), 還有將內(nèi)存中的對象持久化到數(shù)據(jù)庫中時(shí), 好像都沒有去進(jìn)行序列化和反序列化, 因?yàn)槲覀兌紱]有實(shí)現(xiàn)Serializable接口, 但一直正常運(yùn)行.
下面先給出結(jié)論:
只要我們對內(nèi)存中的對象進(jìn)行持久化或網(wǎng)絡(luò)傳輸, 這個(gè)時(shí)候都需要序列化和反序列化.
理由:
服務(wù)器與瀏覽器交互時(shí)真的沒有用到Serializable接口嗎? JSON格式實(shí)際上就是將一個(gè)對象轉(zhuǎn)化為字符串, 所以服務(wù)器與瀏覽器交互時(shí)的數(shù)據(jù)格式其實(shí)是字符串, 我們來看來String類型的源碼:
public?final?class?String
????implements?java.io.Serializable, Comparable<String>, CharSequence?{
????/** The value is used for character storage. */
????private?final?char value[];
????/** Cache the hash code for the string */
????private?int hash; // Default to 0
????/** use serialVersionUID from JDK 1.0.2 for interoperability */
????private?static?final?long serialVersionUID = -6849794470754667710L;
??......
}String類型實(shí)現(xiàn)了Serializable接口, 并顯示指定serialVersionUID的值.
然后我們再來看對象持久化到數(shù)據(jù)庫中時(shí)的情況, ?Mybatis數(shù)據(jù)庫映射文件里的insert代碼:
<insert?id="insertUser"?parameterType="org.tyshawn.bean.User">
????INSERT INTO t_user(name, age) VALUES (#{name}, #{age})
insert>實(shí)際上我們并不是將整個(gè)對象持久化到數(shù)據(jù)庫中, 而是將對象中的屬性持久化到數(shù)據(jù)庫中, 而這些屬性都是實(shí)現(xiàn)了Serializable接口的基本屬性.
實(shí)現(xiàn)序列化和反序列化為什么要實(shí)現(xiàn)Serializable接口?
在Java中實(shí)現(xiàn)了Serializable接口后, JVM會(huì)在底層幫我們實(shí)現(xiàn)序列化和反序列化, 如果我們不實(shí)現(xiàn)Serializable接口, 那自己去寫一套序列化和反序列化代碼也行, 至于具體怎么寫, Google一下你就知道了.
實(shí)現(xiàn)Serializable接口就算了, 為什么還要顯示指定serialVersionUID的值?
如果不顯示指定serialVersionUID, JVM在序列化時(shí)會(huì)根據(jù)屬性自動(dòng)生成一個(gè)serialVersionUID, 然后與屬性一起序列化, 再進(jìn)行持久化或網(wǎng)絡(luò)傳輸. 在反序列化時(shí), JVM會(huì)再根據(jù)屬性自動(dòng)生成一個(gè)新版serialVersionUID, 然后將這個(gè)新版serialVersionUID與序列化時(shí)生成的舊版serialVersionUID進(jìn)行比較, 如果相同則反序列化成功, 否則報(bào)錯(cuò).
如果顯示指定了serialVersionUID, JVM在序列化和反序列化時(shí)仍然都會(huì)生成一個(gè)serialVersionUID, 但值為我們顯示指定的值, 這樣在反序列化時(shí)新舊版本的serialVersionUID就一致了.
在實(shí)際開發(fā)中, 不顯示指定serialVersionUID的情況會(huì)導(dǎo)致什么問題? 如果我們的類寫完后不再修改, 那當(dāng)然不會(huì)有問題, 但這在實(shí)際開發(fā)中是不可能的, 我們的類會(huì)不斷迭代, 一旦類被修改了, 那舊對象反序列化就會(huì)報(bào)錯(cuò). 所以在實(shí)際開發(fā)中, 我們都會(huì)顯示指定一個(gè)serialVersionUID, 值是多少無所謂, 只要不變就行.
寫個(gè)實(shí)例測試下:
(1) User類
public?class?User?implements?Serializable?{
????private?String name;
????private?Integer age;
????public?String getName()?{
????????return?name;
????}
????public?void?setName(String name)?{
????????this.name = name;
????}
????public?Integer getAge()?{
????????return?age;
????}
????public?void?setAge(Integer age)?{
????????this.age = age;
????}
????@Override
????public?String toString()?{
????????return?"User{"?+
????????????????"name='"?+ name + '\''?+
????????????????", age="?+ age +
????????????????'}';
????}
}(2) 測試類
先進(jìn)行序列化, 再進(jìn)行反序列化.
public?class?SerializableTest?{
????private?static?void?serialize(User user)?throws?Exception {
????????ObjectOutputStream oos = new?ObjectOutputStream(new?FileOutputStream(new?File("D:\\111.txt")));
????????oos.writeObject(user);
????????oos.close();
????}
????private?static?User deserialize()?throws?Exception{
????????ObjectInputStream ois = new?ObjectInputStream(new?FileInputStream(new?File("D:\\111.txt")));
????????return?(User) ois.readObject();
????}
????public?static?void?main(String[] args)?throws?Exception {
????????User user = new?User();
????????user.setName("tyshawn");
????????user.setAge(18);
????????System.out.println("序列化前的結(jié)果: "?+ user);
????????serialize(user);
????????User dUser = deserialize();
????????System.out.println("反序列化后的結(jié)果: "+ dUser);
????}
}(3) 結(jié)果
先注釋掉反序列化代碼, 執(zhí)行序列化代碼, 然后User類新增一個(gè)屬性sex
public?class?User?implements?Serializable?{
????private?String name;
????private?Integer age;
????private?String sex;
????public?String getName()?{
????????return?name;
????}
????public?void?setName(String name)?{
????????this.name = name;
????}
????public?Integer getAge()?{
????????return?age;
????}
????public?void?setAge(Integer age)?{
????????this.age = age;
????}
????public?String getSex()?{
????????return?sex;
????}
????public?void?setSex(String sex)?{
????????this.sex = sex;
????}
????@Override
????public?String toString()?{
????????return?"User{"?+
????????????????"name='"?+ name + '\''?+
????????????????", age="?+ age +
????????????????", sex='"?+ sex + '\''?+
????????????????'}';
????}
}再注釋掉序列化代碼執(zhí)行反序列化代碼, 最后結(jié)果如下:
序列化前的結(jié)果: User{name='tyshawn', age=18}
Exception in?thread "main"?java.io.InvalidClassException: org.tyshawn.SerializeAndDeserialize.User; local class?incompatible: stream?classdesc?serialVersionUID?= 1035612825366363028, local class?serialVersionUID?= -1830850955895931978報(bào)錯(cuò)結(jié)果為序列化與反序列化產(chǎn)生的serialVersionUID不一致.
接下來我們在上面User類的基礎(chǔ)上顯示指定一個(gè)serialVersionUID
private?static?final?long?serialVersionUID = 1L;再執(zhí)行上述步驟, 測試結(jié)果如下:
序列化前的結(jié)果: User{name='tyshawn', age=18}
反序列化后的結(jié)果: User{name='tyshawn', age=18, sex='null'}顯示指定serialVersionUID后就解決了序列化與反序列化產(chǎn)生的serialVersionUID不一致的問題.
Java序列化的其他特性
先說結(jié)論, 被transient關(guān)鍵字修飾的屬性不會(huì)被序列化, static屬性也不會(huì)被序列化.
我們來測試下這個(gè)結(jié)論:
(1) User類
public?class?User?implements?Serializable?{
????private?static?final?long?serialVersionUID = 1L;
????private?String name;
????private?Integer age;
????private?transient?String sex;
????private?static?String signature = "你眼中的世界就是你自己的樣子";
????public?String getName()?{
????????return?name;
????}
????public?void?setName(String name)?{
????????this.name = name;
????}
????public?Integer getAge()?{
????????return?age;
????}
????public?void?setAge(Integer age)?{
????????this.age = age;
????}
????public?String getSex()?{
????????return?sex;
????}
????public?void?setSex(String sex)?{
????????this.sex = sex;
????}
????public?static?String getSignature()?{
????????return?signature;
????}
????public?static?void?setSignature(String signature)?{
????????User.signature = signature;
????}
????@Override
????public?String toString()?{
????????return?"User{"?+
????????????????"name='"?+ name + '\''?+
????????????????", age="?+ age +
????????????????", sex='"?+ sex +'\''?+
????????????????", signature='"?+ signature + '\''?+
????????????????'}';
????}
}(2) 測試類
public?class?SerializableTest?{
????private?static?void?serialize(User user)?throws?Exception {
????????ObjectOutputStream oos = new?ObjectOutputStream(new?FileOutputStream(new?File("D:\\111.txt")));
????????oos.writeObject(user);
????????oos.close();
????}
????private?static?User deserialize()?throws?Exception{
????????ObjectInputStream ois = new?ObjectInputStream(new?FileInputStream(new?File("D:\\111.txt")));
????????return?(User) ois.readObject();
????}
????public?static?void?main(String[] args)?throws?Exception {
????????User user = new?User();
????????user.setName("tyshawn");
????????user.setAge(18);
????????user.setSex("man");
????????System.out.println("序列化前的結(jié)果: "?+ user);
????????serialize(user);
????????User dUser = deserialize();
????????System.out.println("反序列化后的結(jié)果: "+ dUser);
????}
}(3) 結(jié)果
先注釋掉反序列化代碼, 執(zhí)行序列化代碼, 然后修改User類signature = “我的眼里只有你”, 再注釋掉序列化代碼執(zhí)行反序列化代碼, 最后結(jié)果如下:
序列化前的結(jié)果: User{name='tyshawn', age=18, sex='man', signature='你眼中的世界就是你自己的樣子'}
反序列化后的結(jié)果: User{name='tyshawn', age=18, sex='null', signature='我的眼里只有你'}static屬性為什么不會(huì)被序列化?
因?yàn)樾蛄谢轻槍ο蠖缘? 而static屬性優(yōu)先于對象存在, 隨著類的加載而加載, 所以不會(huì)被序列化.
看到這個(gè)結(jié)論, 是不是有人會(huì)問, serialVersionUID也被static修飾, 為什么serialVersionUID會(huì)被序列化? 其實(shí)serialVersionUID屬性并沒有被序列化, JVM在序列化對象時(shí)會(huì)自動(dòng)生成一個(gè)serialVersionUID, 然后將我們顯示指定的serialVersionUID屬性值賦給自動(dòng)生成的serialVersionUID.
原文鏈接：csdn.net/litianxiang_kaola/article/details/100097019