How to compile Java to WASM ( WebAssembly )?

Compiling Java into WebAssembly (WASM) is a complex process because WebAssembly is a low-level bytecode format, while Java is a high-level language running on the JVM (Java Virtual Machine). However, there are methods and tools that can help you achieve this.

Using TeaVM

One popular method is to use TeaVM, which is a compiler that converts Java bytecode to JavaScript and also supports compiling Java into WebAssembly.

1. Add TeaVM Dependency

   First, add the TeaVM dependency to your Java project. If your project is a Maven project, you can add the following dependency to your pom.xml file:

   xml
   <dependency>
       <groupId>org.teavm</groupId>
       <artifactId>teavm-platform</artifactId>
       <version>0.6.1</version>
   </dependency>
   <dependency>
       <groupId>org.teavm</groupId>
       <artifactId>teavm-backend-wasm</artifactId>
       <version>0.6.1</version>
   </dependency>

2. Configure TeaVM

   Next, configure TeaVM to generate WebAssembly. This requires setting the target directory and other relevant configurations. If using Maven, you can configure the TeaVM plugin in your pom.xml file:

   xml
   <build>
       <plugins>
           <plugin>
               <groupId>org.teavm</groupId>
               <artifactId>teavm-maven-plugin</artifactId>
               <version>0.6.1</version>
               <executions>
                   <execution>
                       <goals>
                           <goal>compile</goal>
                       </goals>
                       <configuration>
                           <targetType>webassembly</targetType>
                           <mainClass>com.example.Main</mainClass>
                       </configuration>
                   </execution>
               </executions>
           </plugin>
       </plugins>
   </build>

   In the above configuration, com.example.Main refers to the class containing the public static void main(String[] args) method, which is the entry point of the Java program.

3. Compile the Project

   Use the Maven command-line tool to compile the project:

   sh
   mvn clean package

   After compilation, you will receive an output containing WebAssembly and JavaScript glue code, which can be executed in a web environment.

Using Other Tools

Besides TeaVM, there are other tools and approaches you can try, such as:

• JWebAssembly: A library that converts Java bytecode to WebAssembly.
• Bytecoder: This project allows you to compile Java bytecode into WebAssembly and also supports other languages like Kotlin.

Important Considerations

• When compiling Java into WASM, note that many features in the Java Standard Library may not be available or require special handling in the WASM environment.
• Certain Java features, such as multithreading, may not be usable in the current WebAssembly version. WebAssembly's multithreading support is actively being developed but is not yet widely available.
• Performance and size issues: Java applications using WebAssembly may not achieve the performance level of native Java applications, and the generated files may be quite large due to the inclusion of parts of the Java runtime.

Before proceeding, it is recommended to thoroughly read the documentation of the relevant tools to understand how to configure and use them, as well as their limitations and best practices.