Runtime execution of embedded shared archive (ESA) files.

Proto4j-ESA is a code generation and transformation library that can create embedded shared archive (ESA) files that can be loaded and executed during the runtime of a Java application. This project is not limited to Java applications but can also be integrated in the building process of an Android app. Furthermore, Proto4j-ESA offers a convenient API to interact with runtime-defined classes (invoke methods, query fields and create instances).

In order to use/create an embedded shared archive, one does not require an understanding of Java byte code or the class file format. To include classes in a generated embedded JAR file, they just have to be annotated with an annotation. In addition, the destination class, where all contents of the embedded JAR file will be stored, can be chosen by annotating it with a special annotation.

Proto4j-ESA is written in Java 8 and can be marked as a leigh-weight library, which only depends on the visitor API of the Java byte code parser library ASM and the Proto4j-Dx-API in order to create DEX-files for Android projects. Note that the Proto4j-Dx dependency is only used when developing the Gradle-Plugin - the main API of this project does not need this dependency to be integrated.

The main goal of this repository was to create a simple API that can generate an encrypted JAR file with classes that have been marked to be included in the output JAR file. If the compilation process has finished, the marked classes will be transformed and finally wrapped into the final JAR. As this step is part of the general building process, a simple Gradle-Plugin has been developed with Groovy.

When speaking of an embedded shared archive (ESA), an encrypted JAR file that is stored inside a small Java class file is referenced. Although, an ESA does not contain any other resources than the standard manifest file by default, it is possible that a generated DEX-file is stored inside. It is required when working on Android projects. Generated and encrypted JAR files will be stored in Java classes that implement the ESAFile interface.

To encrypt generated JAR files, a custom implementation of an AES-Cipher will be used. It is also possible to use your own cipher implementations, because the default one does symmetric crypto operations (one key). Make sure, the ICipher implementation is available for both, runtime environment and gradle-plugin.

The ESA file will be placed either in a class annotated with Output or in the default class, named defpackage/JarContent. This repository aims to simplify the process of generation with the least amount of limitations to the user. To define the output class, just put the desired annotation on it:

@Output
class Foo implements Serializable {}

The created class can store any kind of data and can extend any other class, but the following fields and methods shouldn't be implemented before generation:

• Fields: filename (psf String) and encodedJar (psf String) used to store the filename and ESA
• Methods: getFilename() and getEncoded(); two methods that will be implemented from the ESAFile interface after the transformation.

As mentioned above, the class annotated with Output will implement the ESAFile interface on a successful generation. The class defined before would look like this:

// All other interfaces that have been implemented before will
// be added as well
class Foo implements ESAFile, Serializable {
    // These fields will be generated
    public static final String filename = "...";
    public static final String encodedJar = "...";

    @Override // automatic method
    public String getFilename() {return filename;}

    // The needed stack size for this method will be calculated as well
    @Override
    public String getEncoded() {return encodedJar;}
}

Finally, a esa object has to be created (in order to interact with classes that have been marked as shared). Every shared Jar can be configured with the following options:

As a result, the final code you would write to create a default ESA instance would look like this:

ESA esa = new DefaultBuilder()
        // can be any class implementing JarConfiguration
        .configure(new MyConfiguration())
        // class annotated with @Output
        .setOutputObject(new Foo())
        // unsafe, DO NOT DO THAT
        .setProvider(new PlainTextKeyProvider("SuperSecretPassw", "AES")) 
        .finish();

Receiving Hello World by invoking a shared method is rather simple. The first step of each project is to decide which classes are shared, so they will be invisible at runtime. In this case, the class MessageUtil will be shared:

package org.example; // Package declaration important later on

@Shadow // This annotation makes the class invisible at runtime 
class MessageUtil {
    public static String getMessage() {
        return "Hello World!";
    }
}

Make sure all references to this class are removed, otherwise ClassDefNotFound exceptions would be thrown by the build-in class loader. There are two ways how to execute the shared method:

1. Write a class that extends the SharedMethodExecutor and sets all required fields automatically. The Encrypt annotation together with ESA.wrap() can be used to encrypt the shared class references at runtime. For more information on how the wrap is done, refer to the Wiki of this project.

   // Usually, custom executors override the invoke(Method) function as the target
   // method contains arguments and/or is public/private (not static).
   class HelloWorldExecutor extends SharedMethodExecutor<String> {
        
        @Encrypt("org.example.Message")  // the string will be encrypted,
        private static final String className = ESA.wrap();
   
        // so it will be decrypted before searching the method
        @Encrypt("getMessage") 
        private static final String methodName = ESA.wrap();
   
        public HelloWorldExecutor(ESA jar) {
            super(jar);
            // Set all required fields of this executor. As we have no arguments
            // on the getMessage() method, the argTypes field has to be an empty
            // class array.
            targetClassName = className;
            targetMethodName = methodName;
            argTypes = new Class[0];
        }
   }  

2. Create an instance of the shared method executor directly, only if the method is static and has no arguments:

   ESA esa = /*see code above*/;
   SharedExecutor<String> executor = new SharedMethodExecutor(
       esa, String.class, // shared JAR and return type
       // the reference name doesn't has to be encrypted
       "org.example.MessageUtil", 
       "getMessage", new Class[0]
   );
   
   try {
       String message = executor.call();
       // handle the received message
   } catch (SharedException e) {
       // handle errors
   }    

Instructions follow...

f you have a suggestion that would make this better, please fork the repo and create a pull request. You can also simply open an issue with the tag "enhancement".

1. Fork the Project
2. Create your Feature Branch (git checkout -b feature/featureName)
3. Commit your Changes (git commit -m 'Add some Feature')
4. Push to the Branch (git push origin feature/featureName)
5. Open a Pull Request

Distributed under the terms of the Apache License 2.0.