Expand description
Java Deserializer for Rust
Java has a much maligned but still widely used Serialization mechanism. The serial stream produced follows the specification available from Oracle here (link is to Java 17 (latest LTS version at time of writing) but protocol hasn’t changed since 1.7).
This library enables that serial stream to be read in Rust applications.
Example
A simple bean type Java class can be serialized using the builtin tools
import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
public class Demo implements Serializable {
private static final long serialVersionUID = 1L;
private String message;
private int i;
public Demo(String message, int count) {
this.message = message;
this.i = count;
}
public static void main(String[] args) throws Exception {
Demo d = new Demo("helloWorld", 42);
try (FileOutputStream fos = new FileOutputStream("demo.obj", false);
ObjectOutputStream oos = new ObjectOutputStream(fos);) {
oos.writeObject(d);
}
}
}
We can read in the demo.obj file written as a rust struct
// Open the file written in Java
let sample = File::open("demo.obj").expect("File missing");
// Create a new parser to wrap the file
let mut parser = Parser::new(sample)?;
// read an object from the stream
let content = parser.read()?;
// the content read was a value (instead of raw data) and the value
// was an instance of an object. These methods would panic if the content
// was of a different type (equivalent to Option#unwrap).
let demo = content.value().object_data();
assert_eq!("Demo", demo.class_name());
assert_eq!(Some(JavaString("helloWorld".to_string())).as_ref(), demo.get_field("message"));
assert_eq!(Some(Primitive(Int(42))).as_ref(), demo.get_field("i"));Reading into the raw data format is not often very user friendly so rust
types can be read directly from the stream if they implement FromJava.
This is implemented for the primitive types and common types (boxed primitives,
arrays, String). Custom types can use these to implement FromJava
themselves.
For example to read a string from an object stream
let mut parser = Parser::new(sample)?;
let string: String = parser.read_as()?;Implemententing FromJava for custom types is very repetitive so a
derive macro is provided with the derive feature to
automatically generate implementations. Using the same Demo class from
above this gives us
#[derive(Debug, FromJava)]
struct Demo {
message: String,
i: i32,
}
let mut parser = Parser::new(sample)?;
let demo: Demo = parser.read_as()?;
println!("{:#?}", demo);
// Output
// Demo {
// message: "helloWorld",
// i: 42,
// }