I have a problem on Scala. I serialize an instance of class with @transient lazy val field. And then I deserialize it, the field is assigned null. I expect the lazy evaluation after deserialization. What should I do?
Following is a sample code.
object Test {
def main(args: Array[String]){
//----------------
// ClassA - with @transient
//----------------
val objA1 = ClassA("world");
println(objA1);
// This works as expected as follows:
// "Good morning."
// "Hello, world"
saveObject("testA.dat", objA1);
val objA2 = loadObject("testA.dat").asInstanceOf[ClassA];
println(objA2);
// I expect this will work as follows:
// "Good morning."
// "Hello, world"
// but actually it works as follows:
// "null"
//----------------
// ClassB - without @transient
// this works as expected
//----------------
val objB1 = ClassB("world");
println(objB1);
// This works as expected as follows:
// "Good morning."
// "Hello, world"
saveObject("testB.dat", objB1);
val objB2 = loadObject("testB.dat").asInstanceOf[ClassB];
println(objB2);
// This works as expected as follows:
// "Hello, world"
}
case class ClassA(name: String){
@transient private lazy val msg = {
println("Good morning.");
"Hello, " + name;
}
override def toString = msg;
}
case class ClassB(name: String){
private lazy val msg = {
println("Good morning.");
"Hello, " + name;
}
override def toString = msg;
}
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
def saveObject(fname: String, obj: AnyRef){
val fop = new FileOutputStream(fname);
val oop = new ObjectOutputStream(fop);
try {
oop.writeObject(obj);
} finally {
oop.close();
}
}
def loadObject(fname: String): AnyRef = {
val fip = new FileInputStream(fname);
val oip = new ObjectInputStream(fip);
try {
oip.readObject();
} finally {
oip.close();
}
}
}
Kevin Wright
来源：https://stackoverflow.com/questions/4772825/transient-lazy-val-field-serialization