j4rs stands for 'Java for Rust' and allows effortless calls to Java code from Rust and vice-versa.
j4rs focused solely in facilitating Rust applications in making calls to Java code
by allowing JVM creation and manipulation from the Rust code, efortless Java method invocations,
Java libraries provisioning via Maven and more.
This has changed since release 0.12.0. j4rs can now be used as well in Java projects that want to achieve JNI calls to Rust libraries.
```rust use j4rs::{Instance, InvocationArg, Jvm, JvmBuilder};
// Create a JVM let jvm = JvmBuilder::new().build()?;
// Create a java.lang.String instance
let stringinstance = jvm.createinstance(
"java.lang.String", // The Java class to create an instance for
&Vec::new(), // The InvocationArgs to use for the constructor call - empty for this example
)?;
// The instances returned from invocations and instantiations can be viewed as pointers to Java Objects.
// They can be used for further Java calls.
// For example, the following invokes the isEmpty method of the created java.lang.String instance
let booleaninstance = jvm.invoke(
&stringinstance, // The String instance created above
"isEmpty", // The method of the String instance to invoke
&Vec::new(), // The InvocationArgs to use for the invocation - empty for this example
)?;
// If we need to transform an Instance to Rust value, the to_rust should be called
let rustboolean: bool = jvm.torust(booleaninstance)?;
println!("The isEmpty() method of the java.lang.String instance returned {}", rustboolean);
// The above prints:
// The isEmpty() method of the java.lang.String instance returned true
// Static invocation
let staticinvocationresult = jvm.invokestatic(
"java.lang.System", // The Java class to invoke
"currentTimeMillis", // The static method of the Java class to invoke
&Vec::new(), // The InvocationArgs to use for the invocation - empty for this example
)?;
```
j4rs uses the InvocationArg enum to pass arguments to the Java world.
Users can benefit of the existing TryFrom implementations for several basic types:
rust
let i1 = InvocationArg::try_from("a str")?; // Creates an arg of java.lang.String
let my_string = "a string".to_owned();
let i2 = InvocationArg::try_from(my_string)?; // Creates an arg of java.lang.String
let i3 = InvocationArg::try_from(true)?; // Creates an arg of java.lang.Boolean
let i4 = InvocationArg::try_from(1_i8)?; // Creates an arg of java.lang.Byte
let i5 = InvocationArg::try_from('c')?; // Creates an arg of java.lang.Character
let i6 = InvocationArg::try_from(1_i16)?; // Creates an arg of java.lang.Short
let i7 = InvocationArg::try_from(1_i64)?; // Creates an arg of java.lang.Long
let i8 = InvocationArg::try_from(0.1_f32)?; // Creates an arg of java.lang.Float
let i9 = InvocationArg::try_from(0.1_f64)?; // Creates an arg of java.lang.Double
And for Vecs:
```rust
let myvec: Vec
let i10 = InvocationArg::tryfrom(myvec.as_slice())?; ```
The Instances returned by j4rs can be transformed to InvocationArgs and be further used for invoking methods as well:
``rust
let one_more_string_instance = jvm.create_instance(
"java.lang.String", // The Java class to create an instance for
&Vec::new(), // TheInvocationArg`s to use for the constructor call - empty for this example
)?;
let i11 = InvocationArg::tryfrom(onemorestringinstance)?; ```
To create an InvocationArg that represents a null Java value, use the From implementation with the Null struct:
rust
let null_string = InvocationArg::from(Null::String); // A null String
let null_integer = InvocationArg::from(Null::Integer); // A null Integer
let null_obj = InvocationArg::from(Null::Of("java.util.List")); // A null object of any other class. E.g. List
An Instance may be casted to some other Class:
rust
let instantiation_args = vec![InvocationArg::try_from("Hi")?];
let instance = jvm.create_instance("java.lang.String", instantiation_args.as_ref())?;
jvm.cast(&instance, "java.lang.Object")?;
```rust // Create a Java array of Strings let s1 = InvocationArg::tryfrom("string1")?; let s2 = InvocationArg::tryfrom("string2")?; let s3 = InvocationArg::try_from("string3")?;
let arrinstance = jvm.createjavaarray("java.lang.String", &vec![s1, s2, s3])?;
// Invoke the Arrays.asList(...) and retrieve a java.util.List
rust
// Assuming that the following map_instance is a Map<String, Integer>
// we may invoke its put method
jvm.invoke(&map_instance, "put", &vec![InvocationArg::try_from("one")?, InvocationArg::try_from(1)?])?;
Even if auto boxing and unboxing is in place, j4rs cannot invoke methods with primitive int arguments using Integer instances.
For example, the following code does not work:
rust
let ia = InvocationArg::try_from(1_i32)?;
jvm.create_instance("java.lang.Integer", &[ia])?;
It throws an InstantiationException because the constructor of Integer takes a primitive int as an argument:
Exception in thread "main" org.astonbitecode.j4rs.errors.InstantiationException: Cannot create instance of java.lang.Integer at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.instantiate(NativeInstantiationImpl.java:37) Caused by: java.lang.NoSuchMethodException: java.lang.Integer.
(java.lang.Integer) at java.base/java.lang.Class.getConstructor0(Class.java:3349) at java.base/java.lang.Class.getConstructor(Class.java:2151) at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.createInstance(NativeInstantiationImpl.java:69) at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.instantiate(NativeInstantiationImpl.java:34)
In situations like this, the java.lang.Integer instance should be transformed to a primitive int first:
rust
let ia = InvocationArg::try_from(1_i32)?.into_primitive()?;
jvm.create_instance("java.lang.Integer", &[ia]);
```rust use j4rs::{Instance, InvocationArg, Jvm, JvmBuilder};
// Create a JVM let jvm = JvmBuilder::new().build()?;
// Create an instance let stringinstance = jvm.createinstance( "java.lang.String", &vec![InvocationArg::try_from(" a string ")?], )?;
// Perform chained operations on the instance let stringsize: isize = jvm.chain(stringinstance) .invoke("trim", &[])? .invoke("length", &[])? .to_rust()?;
// Assert that the string was trimmed assert!(string_size == 8); ```
j4rs provides support for Java to Rust callbacks.
These callbacks come to the Rust world via Rust Channels.
In order to initialize a channel that will provide Java callback values, the Jvm::invoke_to_channel should be called. It returns a result of InstanceReceiver struct, which contains a Channel Receiver:
```rust // Invoke of a method of a Java instance and get the returned value in a Rust Channel.
// Create an Instance of a class that supports Native Callbacks
// (the class just needs to extend the
// org.astonbitecode.j4rs.api.invocation.NativeCallbackToRustChannelSupport)
let i = jvm.create_instance(
"org.astonbitecode.j4rs.tests.MyTest",
&Vec::new())?;
// Invoke the method
let instancereceiverres = jvm.invoketochannel(
&i, // The instance to invoke asynchronously
"performCallback", // The method to invoke asynchronoysly
&Vec::new() // The InvocationArgs to use for the invocation - empty for this example
);
// Wait for the response to come let instancereceiver = instancereceiverres?; let _ = instancereceiver.rx().recv(); ```
In the Java world, a Class that can do Native Callbacks must extend the
org.astonbitecode.j4rs.api.invocation.NativeCallbackToRustChannelSupport
For example, consider the following Java class.
The performCallback method spawns a new Thread and invokes the doCallback method in this Thread. The doCallback method is inherited by the NativeCallbackToRustChannelSupport class.
```java package org.astonbitecode.j4rs.tests;
import org.astonbitecode.j4rs.api.invocation.NativeCallbackToRustChannelSupport;
public class MyTest extends NativeCallbackToRustChannelSupport {
public void performCallback() {
new Thread(() -> {
doCallback("THIS IS FROM CALLBACK!");
}).start();
}
} ```
Since release 0.6.0 there is the possibility to download Java artifacts from the Maven repositories. While it is possible to define more repos, the maven central is by default and always available.
For example, here is how the dropbox dependency can be downloaded and get deployed to be used by the rust code:
rust
let dbx_artifact = MavenArtifact::from("com.dropbox.core:dropbox-core-sdk:3.0.11");
jvm.deploy_artifact(dbx_artifact)?;
Additional artifactories can be used as well:
```rust let jvm: Jvm = JvmBuilder::new() .withmavensettings(MavenSettings::new(vec![ MavenArtifactRepo::from("myrepo1::https://my.repo.io/artifacts"), MavenArtifactRepo::from("myrepo2::https://my.other.repo.io/artifacts")]) ) .build() ?;
jvm.deploy_artifact(&MavenArtifact::from("io.my:library:1.2.3"))?; ```
Maven artifacts are added automatically to the classpath and do not need to be explicitly added.
A good practice is that the deployment of maven artifacts is done by build scripts, during the crate's compilation. This ensures that the classpath is properly populated during the actual Rust code execution.
Note: the deployment does not take care the transitive dependencies yet.
If we have one jar that needs to be accessed using j4rs, we need to add it in the classpath during the JVM creation:
rust
let entry = ClasspathEntry::new("/home/myuser/dev/myjar-1.0.0.jar");
let jvm: Jvm = JvmBuilder::new()
.classpath_entry(entry)
.build()?;
The jar for j4rs is available in the Maven Central. It may be used by adding the following dependency in a pom:
xml
<dependency>
<groupId>io.github.astonbitecode</groupId>
<artifactId>j4rs</artifactId>
<version>0.12.0</version>
<scope>provided</scope>
</dependency>
Note that the scope is provided. This is because the j4rs Java resources are always available with the j4rs crate.
Use like this in order to avoid possible classloading errors.
If you encounter any issues when using j4rs in Android, this may be caused by Java 8 compatibility problems. This is why there is a Java 7 version of j4rs:
xml
<dependency>
<groupId>io.github.astonbitecode</groupId>
<artifactId>j4rs</artifactId>
<version>0.12.0-java7</version>
</dependency>
Add the two needed dependencies (j4rs and j4rs_derive) in the Cargo.toml
and mark the project as a cdylib, in order to have a shared library as output.
This library will be loaded and used by the Java code to achieve JNI calls.
Annotate the functions that will be accessible from the Java code with the call_from_java attribute:
```rust
fn myfunctionwithnoargs() { println!("Hello from the Rust world!"); // If you need to have a Jvm here, you need to attach the thread let jvm = Jvm::attach_thread().unwrap(); // Now you may further call Java classes and methods as usual! } ```
For a complete example, please have a look here.
At your option, under: