How to generate a class at runtime

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Is there a library that facilitates classes code generation at runtime?
Apr 15, 2020 in Java by Edvard
• 180 points

2 answers to this question.

+1 vote
Best answer
Yes, I recently met this library which among other things helps to create classes at runtime: here is the detailed example
answered Apr 15, 2020 by Edureka
• 520 points

selected Apr 15, 2020 by Edvard
+1 vote

Hey, @Edvard,

Dynamic Class creation enables you to create a Java class on the fly at runtime, from source code created from a string. Dynamic class creation can be used in extremely low latency applications to improve performance. Although compiling the source into a class is slow, the newly created class will be treated by the JVM just like any other class.

This is an example to create and used a dynamically created class without creating any intermediate files.

First the source is created using string builder

final String className = "HelloWorld";
final String path = "com.bounded.buffer";
final String fullClassName = path.replace('.', '/') + "/" + className;
final StringBuilder source = new StringBuilder();
source.append("package " + path+";");
source.append("public class " + className + " {\n");
source.append(" public String toString() {\n");
source.append("     return \"HelloWorld - Java Dynamic Class Creation...";");
source.append(" }\n");

This is where the magic happens - the StringBuilder java source, is turned into byte code :

final ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

final SimpleJavaFileObject simpleJavaFileObject
      = new SimpleJavaFileObject(URI.create(fullClassName + ".java"), SOURCE) {

        public CharSequence getCharContent(boolean ignoreEncodingErrors) {
            return source;

       public OutputStream openOutputStream() throws IOException{
           return byteArrayOutputStream;

final JavaFileManager javaFileManager = new ForwardingJavaFileManager(
    getStandardFileManager(null, null, null)) {

        public JavaFileObject getJavaFileForOutput(
            Location location,String className,
            JavaFileObject.Kind kind,
            FileObject sibling) throws .. {
           return simpleJavaFileObject;

ToolProvider.getSystemJavaCompiler().getTask(null, javaFileManager, null, null, null, singletonList(simpleJavaFileObject)).call();

then the bytes that make up the class are loaded into the class loader :

final byte[] bytes = byteArrayOutputStream.toByteArray(); 

// use the unsafe class to load in the class bytes
final Field f = Unsafe.class.getDeclaredField("theUnsafe");

final Unsafe unsafe = (Unsafe) f.get(null);
final Class aClass = unsafe.defineClass(fullClassName, bytes, 0, bytes.length);

and instantiated :

final Object o = aClass.newInstance();

When run, the console will output "HelloWorld 

answered Apr 15, 2020 by Gitika
• 65,910 points

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