反序列化时恢复transient字段
我们知道将字段设置为transient，可以避免该自动被序列化，但若反序列化时又希望该transient有值怎么办呢？一种不使用transient的办法是使用Externalizable接口。Serializable接口默认序列化所有字段，而Externalizable接口继承自Serializable，要求实现两个方法readExternal(ObjectInput),writeExternal(ObjectOutput),默认不序列化任何字段，字段需要显式序列化。显然transient只能和Serializable接口共用，在Serializable类中反序列化时恢复transient的值。可以通过在类中添加private void readObject(ObjectInputStream ois);private void writeObject(ObjectOutputStream oos);两个方法。注：方法必须为private，此方法也非Serializable接口中的方法，但在序列化和反序列化时会自动被调用（很特殊吧）。完整例子如下?public class TestSerializable { ??? public static void main(String[] args) {
??????? String url = "jdbc:oracle:thin:@10.141.245.123:1521:FDU";
??????? Properties prop = new Properties();
??????? prop.setProperty("user", "system");
??????? prop.setProperty("password", "fducs");
??????? SerialObject serialObj = new SerialObject(url,prop);
??????? try {
??????????? ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(new java.io.File("SerialObject.out")));
??????????? oos.writeObject(serialObj);
??????? } catch (IOException ex) {
??????????? Logger.getLogger(TestSerializable.class.getName()).log(Level.SEVERE, null, ex);
??????? }
??????? System.out.println("SerialObject original:\n"+ serialObj); ??????? SerialObject backObj = null;
??????? try {
??????????? ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new java.io.File("SerialObject.out")));
??????????? backObj = (SerialObject) ois.readObject();
??????? } catch (ClassNotFoundException ex) {
??????????? Logger.getLogger(TestSerializable.class.getName()).log(Level.SEVERE, null, ex);
??????? } catch (IOException ex) {
??????????? Logger.getLogger(TestSerializable.class.getName()).log(Level.SEVERE, null, ex);
??????? } ??????? System.out.println("SerialObject target:\n"+ backObj); ??? }
} class SerialObject implements java.io.Serializable { ??? private String url;
??? private Properties prop;
??? private transient Connection con; ??? public SerialObject(String url, Properties prop) {
??????? this.url = url;
??????? this.prop = prop;
??????? con = getConnection(url, prop);
??? } ??? private Connection getConnection(String url, Properties prop) {
??????? Connection connect = null;
??????? try {
??????????? Class.forName("oracle.jdbc.driver.OracleDriver");
??????????? connect = DriverManager.getConnection(url, prop);
??????? } catch (SQLException ex) {
??????????? Logger.getLogger(TestSerializable.class.getName()).log(Level.SEVERE, null, ex);
??????? } catch (ClassNotFoundException ex) {
??????????? Logger.getLogger(TestSerializable.class.getName()).log(Level.SEVERE, null, ex);
??????? }
??????? return connect;
??? } ??? @Override
??? public String toString(){
??????? StringBuffer sbuf = new StringBuffer();
??????? sbuf.append("URL="+url);
??????? sbuf.append("\n");
??????? sbuf.append("Properties="+prop);
??????? sbuf.append("\n");
??????? sbuf.append("Connection="+con);
??????? return sbuf.toString();
??? } ??? private void readObject(ObjectInputStream ois){
??????? try {
??????????? ois.defaultReadObject();
??????????? System.out.println("url="+url);
??????????? System.out.println("prop="+prop);
??????????? con = getConnection(url,prop);
??????? } catch (IOException ex) {
??????????? Logger.getLogger(SerialObject.class.getName()).log(Level.SEVERE, null, ex);
??????? } catch (ClassNotFoundException ex) {
??????????? Logger.getLogger(SerialObject.class.getName()).log(Level.SEVERE, null, ex);
??????? } ??? } ??? private void writeObject(ObjectOutputStream oos){
??????? try {
??????????? oos.defaultWriteObject();
??????? } catch (IOException ex) {
??????????? Logger.getLogger(SerialObject.class.getName()).log(Level.SEVERE, null, ex);
??????? }
??? }
}