j4rs stands for 'Java for Rust' and allows effortless calls to Java code, from Rust.
```rust use j4rs::{Instance, InvocationArg, Jvm, JvmBuilder};
// Create a JVM let jvm = JvmBuilder::new().build().unwrap();
// 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
).unwrap();
// 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
).unwrap();
// If we need to transform an Instance to Rust value, the to_rust should be called
let rustboolean: bool = jvm.torust(booleaninstance).unwrap();
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
).unwrap();
```
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())
.unwrap();
// 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.unwrap(); 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();
}
} ```
j4rs uses the InvocationArg enum to pass arguments to the Java world.
Users can benefit of the existing From implementations for several basic types:
rust
let i1 = InvocationArg::from("a str"); // Creates an arg of java.lang.String
let my_string = "a string".to_owned();
let i2 = InvocationArg::from(my_string); // Creates an arg of java.lang.String
let i3 = InvocationArg::from(true); // Creates an arg of java.lang.Boolean
let i4 = InvocationArg::from(1_i8); // Creates an arg of java.lang.Byte
let i5 = InvocationArg::from('c'); // Creates an arg of java.lang.Character
let i6 = InvocationArg::from(1_i16); // Creates an arg of java.lang.Short
let i7 = InvocationArg::from(1_i64); // Creates an arg of java.lang.Long
let i8 = InvocationArg::from(0.1_f32); // Creates an arg of java.lang.Float
let i9 = InvocationArg::from(0.1_f64); // Creates an arg of java.lang.Double
And for Vecs:
```rust
let myvec: Vec
let i10 = InvocationArg::from((myvec.asslice(), &jvm)); ```
The Instances returned by j4rs can be transformed to InvocationArgs and be 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
).unwrap();
let i11 = InvocationArg::from(onemorestring_instance); ```
An Instance may be casted to some other Class:
rust
let instantiation_args = vec![InvocationArg::from("Hi")];
let instance = jvm.create_instance("java.lang.String", instantiation_args.as_ref()).unwrap();
jvm.cast(&instance, "java.lang.Object").unwrap();
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()
.unwrap();
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>0.4.0</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>0.4.0-java7</version>
</dependency>
$JAVA_HOME is setj4rs as dependency$LD_LIBRARY_PATH is set and includes the directory that contains the libjvm.so shared library (jvm.dll in Windows).The build script attempts to help setting up the environment as needed for the j4rs to work. However, things can be different depending on the use case and the environment.
In a Linux host, the build script of j4rs populates the $LD_LIBRARY_PATH and exports it in the $CARGO_HOME/env file. This means that it is enough to execute
source $CARGO_HOME/env in order to export the correct $LD_LIBRARY_PATH variable in your session.
Implement macros to facilitate j4rs users
Possibly something like:
```rust // Instantiation let i12 = jnew!(&jvm -> new java.lang.String("a-new-string"));
// Invocation let i13 = j!(&i12.split("-")); ```
At your option, under: