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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Microsoft – NPI EA (cat= Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

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With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

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I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.

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End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

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1. Introduction

In Java development, compilation is the first defense against syntax errors, type mismatches, and other issues that can derail a project. While traditional workflows rely on manual compilation, modern applications demand dynamic compilation checks. For instance:

  • An educational platform that validates student code submissions in real time
  • A CI/CD pipeline that compiles generated code snippets before deployment
  • A low-code tool that dynamically compiles user-defined logic
  • A Hot Code Reload system that instantly reloads developer changes
  • Creating Java plugins

The Java Compiler API enables these scenarios by allowing code compilation programmatically within Java applications. Platforms like LeetCode or Codecademy validate user-submitted code instantly. When users click “Run,” the backend compiles the snippet using tools like the Compiler API, checks for errors, and executes it in a sandboxed environment. Programmatic compilation powers this immediate feedback loop.

In this tutorial, we’ll explore how to leverage this powerful tool.

2. Java Compiler API Overview

The Java Compiler API resides within the javax.tools package and provides programmatic access to the Java compiler. This API is crucial for dynamic compilation tasks where code needs to be verified or executed at runtime.

Key components of the Compiler API include:

  • JavaCompiler: The main compiler instance that initiates compilation tasks
  • JavaFileObject: Represents Java source or class files, either in-memory or file-based
  • StandardJavaFileManager: Manages input and output files during the compilation process
  • DiagnosticCollector: Captures compilation diagnostics such as errors and warnings

These components work together to enable flexible and efficient dynamic compilation within Java applications.

Let’s examine how the compilation process works.

3. Step-by-Step: Implementing a Compilation Check

The Compiler API is available by default in JDK environments without needing any external dependencies. Now, let’s see how to compile in-memory Java code from .java files.

3.1. Create an In-Memory Java Source

To compile code stored as a string, we first need to create an in-memory representation of the source file. We do this by extending the SimpleJavaFileObject class:

public class InMemoryJavaFile extends SimpleJavaFileObject {
    private final String code;

    protected InMemoryJavaFile(String name, String code) {
        super(URI.create("string:///" + name.replace('.', '/') + Kind.SOURCE.extension),
              Kind.SOURCE);
        this.code = code;
    }

    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) {
        return code;
    }
}

This class represents the Java code as an in-memory JavaFileObject, enabling us to pass source code directly to the compiler without needing a physical file.

3.2. How the Compile API Works

Next, let’s create a utility method to compile Java code and capture diagnostics:

private boolean compile(Iterable<? extends JavaFileObject> compilationUnits) {
    DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<>();

    JavaCompiler.CompilationTask task = compiler.getTask(
        null, 
        standardFileManager, 
        diagnostics, 
        null, 
        null, 
        compilationUnits
    );

    boolean success = task.call();

    for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
        System.out.println(diagnostic.getMessage(null));
    }

    return success;
}

The compile() method handles Java source compilation through the Compiler API, starting with a DiagnosticCollector to capture compilation messages.

The central compiler.getTask() call accepts six parameters: null for the writer (defaulting to System.err), a standard file manager for handling source files, the diagnostic collector for compilation messages, null for compiler options (using defaults instead of custom flags), null for annotation processing classes (as no specific types need processing), and the provided compilation units containing source files to compile. After executing task.call() , the method logs all diagnostic messages and returns a boolean indicating compilation success.

3.3. Compile From In-Memory String

To make compilation easier to use in client code or test cases, let’s introduce a wrapper method that compiles Java code directly from a String:

public boolean compileFromString(String className, String sourceCode) {
    JavaFileObject sourceObject = new InMemoryJavaFile(className, sourceCode);
    return compile(Collections.singletonList(sourceObject));
}

Here, we create an instance of the InMemoryJavaFile class from earlier and wrap it in a singleton list to pass to the actual compile() method.

3.4. Testing the Compiler

Now that we’ve implemented a method to compile Java code dynamically, let’s test it using both valid and invalid code snippets. This confirms that the API correctly identifies syntax errors and returns appropriate diagnostics:

@Test
void givenSimpleHelloWorldClass_whenCompiledFromString_thenCompilationSucceeds() {
    String className = "HelloWorld";
    String sourceCode = "public class HelloWorld {\n" +
        "    public static void main(String[] args) {\n" +
        "        System.out.println(\"Hello, World!\");\n" +
        "    }\n" +
        "}";
    
    boolean result = compilerUtil.compileFromString(className, sourceCode);
    assertTrue(result, "Compilation should succeed");
    
    // Check if the class file was created
    Path classFile = compilerUtil.getOutputDirectory().resolve(className + ".class");
    assertTrue(Files.exists(classFile), "Class file should be created");
}

This test confirms that the compiler processes and compiles valid Java source code into an executable class file generated in the expected output directory.

Next, we verify error capturing by testing code with a syntax error:

@Test
void givenClassWithSyntaxError_whenCompiledFromString_thenCompilationFails() {
    String className = "ErrorClass";
    String sourceCode = "public class ErrorClass {\n" +
        "    public static void main(String[] args) {\n" +
        "        System.out.println(\"This has an error\")\n" +
        "    }\n" +
        "}";
    
    boolean result = compilerUtil.compileFromString(className, sourceCode);
    assertFalse(result, "Compilation should fail due to syntax error");
    
    Path classFile = compilerUtil.getOutputDirectory().resolve(className + ".class");
    assertFalse(Files.exists(classFile), "No class file should be created for failed compilation");
}

Since the compilation fails, no .class file is created, confirming that errors are properly captured.

4. Conclusion

In this article, we explored the Java Compiler API and its role in programmatic code compilation. We learned how to compile in-memory source code, capture diagnostics, and execute compilation dynamically.

By leveraging the Compiler API, we can:

  • Automate compilation workflows in CI/CD pipelines, educational platforms, and low-code environments
  • Validate and execute user-defined code dynamically within applications
  • Improve debugging and error handling by capturing detailed diagnostics

Whether we’re building an automated grading system, a plugin system, or a tool for dynamic Java execution, the Java Compiler API provides a powerful and flexible solution.

As always, the full source code for this article is over on GitHub.

Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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Course – LS – NPI EA (cat=REST)

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