Java has a much maligned (for good reason) serialization system built into the standard library. The output is a binary stream mapping the full object hierarchy and the relations between them.
The stream also includes definitions of classes and their hierarchies (super classes etc). The full specification is defined here.
In any new application there are probably better ways to serialize data with fewer security risks but there are cases where a legacy application is writing stuff out and we want to read it in again. If we want to read it in a separate application it'd be good if we weren't bound to Java.
java
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);
}
}
}
```rust use std::fs::File; use jaded::{Parser, Result};
fn main() -> Result<()> { let sample = File::open("demo.obj").expect("File missing"); let mut parser = Parser::new(sample)?; println!("Read Object: {:#?}", parser.read()?); Ok(()) } ```
Read Object: Object(
Object(
ObjectData {
class: "Demo",
fields: {
"i": Primitive(
Int(
42,
),
),
"message": JavaString(
"helloWorld",
),
},
annotations: [],
},
),
)
For most uses cases, the raw object representation is not very ergonomic
to work with. For ease of use, types can implement FromJava, and can then be
read directly from the stream.
In the majority of cases this implementation can be automatically derived by
enabling the derive feature.
```rust
struct Demo {
message: String,
i: i32,
}
Demo objects can then be read directly by the parser
rust
fn main() -> Result<()> {
let sample = File::open("demo.obj").expect("File missing");
let mut parser = Parser::new(sample)?;
let demo: Demo = parser.read_as()?;
println!("Read Object: {:#?}", demo);
Ok(())
}
Output from rust
Read Object: Demo {
message: "helloWorld",
i: 42,
}
```
Often classes, including many in the standard library, customise the way they
are written using a writeObject method that complements the builtin serialization
methods for fields. This data is written as an embedded stream of bytes and/or
objects. These cannot be associated with fields without the original Java source
so are included in the annotations field of the ObjectData struct (empty in
the example above).
As this stream often contains important data from the class, a mechanism is provided to read useful data from it using an interface similar to the ObjectInputStream that would be used in the Java class itself.
An example of custom serialization in Java is the ArrayList. The source for its
writeObject methods can be seen
here
but the gist is that it writes the number of elements it contains, then writes
each element in turn.
Because the embedded custom stream could contain anything we have to manually
implement the methods to read from it but these can then be used by the
derived implementation of FromJava:
In Java
java
import java.util.List;
import java.util.ArrayList;
import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
public class Demo {
public static void main(String[] args) throws Exception {
List<String> keys = new ArrayList<>();
keys.add("one");
keys.add("two");
keys.add("three");
try (FileOutputStream fos = new FileOutputStream("demo.obj", false);
ObjectOutputStream oos = new ObjectOutputStream(fos);) {
oos.writeObject(keys);
}
}
}
In rust
```rust
use std::fs::File;
use jaded::{Parser, Result, FromJava, FromJava, AnnotationIter, ConversionResult};
struct ArrayList
// extraction method must be callable as
// function(&mut AnnotationIter) -> ConversionResult where S: Into
fn main() -> Result<()> {
let sample = File::open("demo.obj").expect("File missing");
let mut parser = Parser::new(sample)?;
let array: ArrayList
This gives the array list as expected
ArrayList {
size: 3,
values: [
"one",
"two",
"three",
],
}
```
FromJava is implemented for Option<T> and Box<T> so that recursive structs
can be deserialized and null fields in the serialized class can be handled.
Note that if a field is null the conversion will fail unless that field is
given as Option<T>. The example above would have failed if there was a null
string in the serialized list. Changing values to be Vec<Option<T>>
would allow it to still be read.
In Java conventions, field names use camelCase whereas Rust field names use
snake_case. By default, the derive macro looks for a field named the same
as the mapped field in Rust so to prevent Rust structs needing to use camelCase,
fields can be given attributes to use a different field in the Java class.
```rust
struct Demo { #[jaded(field = "fooBar")] foo_bar: String, } ```
If all fields are to be renamed, the struct can be given a 'rename' attribute. This will convert all field names to camelCase before reading them from Java. Individual fields can still be overridden if required.
```rust
struct Demo { foo_bar: String, } ```
In Java, a field can be declared as an interface and the concrete implementation
cannot be known until runtime. Jaded can go some way towards deserializing
these fields, using the built in derive macro with an enum.
Each variant of the enum can be assigned a concrete implementation and the fully qualified class name (FQCN) of the object being read will determine which variant is returned.
For instance, to read a field declared as a list, you might define an enum as follows
```rust
enum List
Combined with thefromfield attribute and aFromimplementation, this
enables fields declared asListin Java to be read toVec
While this helps to support polymorhpism, it still requires all potential implementations to be known up front. For many use cases this should be adequate.
Allow FromJava to be derived automatically
In Java, a field can be declared as an interface and the concrete implementation
can be anything. This means that in Rust we can't reliably convert read objects
to structs unless we know that a stream is going to be using a specific
implementation.
While deserializing to an enum would cover most of the common cases, there
is nothing stopping some client code creating a CustomList with a
completely different serialized representation and using that in the class that
is being read in Rust.
Unfortunately, there are also limits to what we can do without the original code
that created the serial byte stream. The protocol linked above lists four types
of object. One of which, classes that implement java.lang.Externalizable and
use PROTOCOLVERSION1 (not been the default since v1.2), are not readable by
anything other than the class that wrote them as their data is nothing more
than a stream of bytes.
Of the remaining three types we can only reliably deserialize two.
'Normal' classes that implement java.lang.Serializable without having
a writeObject method
These can be read as shown above
Classes that implement Externalizable and use the newer PROTOCOLVERSION2
These can be read, although their data is held fully by the annotations fields of the ObjectData struct and the get_field method only returns None.
Serializable classes that implement writeObject
These objects are more difficult. The spec above suggests that they have their fields written as 'normal' classes and then have optional annotations written afterwards. In practice this is not the case and the fields are only written if the class calls defaultWriteObject as the first call in their writeObject method. This is mentioned as a requirement in the spec so we can assume that this is correct for classes in the standard library but it is something to be aware of if user classes are being deserialized.
A consequence of this is that once we have found a class that we can't read, it is difficult to get back on track as it requires picking out the marker signifying the start of the next object from the sea of custom data.
FromJava for common Java and Rust types so that
for instance ArrayList and HashMap can be read to the equivalent Vec
and HashMap types in Rust.read on a parser
instance builds the next object by cloning any data referenced by the next
object and returning that. This can lead to the same data being cloned many
times. It would be better to keep an internal pool of read objects and return
objects built of references to that pool. This would mean that read could
not be called while a reference to a returned object was still held but if
required, the clone could be made on the client side.Very much a work in progress at the moment. I am writing this for another application I am working on so I imagine there will be many changes in the functionality and API at least in the short term as the requirements become apparent. As things settle down I hope things will become more stable.
As this project it is still very much in a pre-alpha state, I imagine things being quite unstable for a while. That said, if you notice anything obviously broken or have a feature that you think would be useful that I've missed entirely, do open issues. I'd avoid opening PRs until it's been discussed in an issue as the current repo state may lag behind development.