Introduction to the Class-File API

1. Introduction

The Java Class-File API was introduced in JEP-484 as part of Java 24. It aims to create an interface that allows class file processing without relying on the legacy JDK’s internal copy implementation of the ASM library.

In this tutorial, we’ll look at how to build class files from scratch and how to transform a class file into another using the Class-File API.

2. Core Class-File API Components

The Class-File has three core elements to generate and transform features that we’ll see later:

• An element represents a part of the code, like a variable, an instruction, a method, or a class. Additionally, one element might contain other elements. For instance, a class element may contain method elements, which include variable or instruction elements.
• Builders, such as method builders and code builders, are used to create each type of element.
• A transform function can be used to transform elements into other elements using builders.

Let’s explore how these three components connect through practical examples in the following sections.

3. Generating a Class-File

In this section, we’ll see how to generate a class file using the MethodBuilder and CodeBuilder classes.

3.1. Demo Method

To illustrate class generation, let’s look at a simple code snippet that calculates an employee’s salary based on their function and base salary:

public double calculateAnnualBonus(double baseSalary, String role) {
    if (role.equals("sales")) {
        return baseSalary * 0.35;
    }

    if (role.equals("engineer")) {
        return baseSalary * 0.25;
    }

    return baseSalary * 0.15;
}

3.2. Using MethodBuilder and CodeBuilder

To generate a method with the same functionality as calculateAnnualBonus(), we can use the MethodBuilder and CodeBuilder classes. Hence, let’s first define the generate() method with a Consumer<MethodBuilder> that will be used to construct methods:

public static void generate() throws IOException {
    Consumer<MethodBuilder> calculateAnnualBonusBuilder = methodBuilder -> methodBuilder.withCode(codeBuilder -> {
        Label notSales = codeBuilder.newLabel();
        Label notEngineer = codeBuilder.newLabel();
        ClassDesc stringClass = ClassDesc.of("java.lang.String");

        // ...
    });
}

First, we define two Label objects that will be used later for jumping between conditional statements. Additionally, we’ve defined a ClassDesc constant that represents the String class file for later use.

Then, we can add the first part of our logic inside the calculateAnnualBonusBuilder‘s lambda expression:

codeBuilder.aload(3)
  .ldc("sales")
  .invokevirtual(stringClass, "equals", MethodTypeDesc.of(ClassDesc.of("Z"), stringClass))
  .ifeq(notSales)
  .dload(1)
  .ldc(0.35)
  .dmul()
  .dreturn()

Let’s look at each line of the above logic in detail:

• We first start using aload(3) to load the role method parameter into a reference. Noticeably, the parameter of aload() is the slot number of the variable in the method arguments, where long and double take two slots. Thus, the first baseSalary argument is at slot 1, and role is at slot 3.
• Then, we use ldc() to store the sales constant in the operand stack for the subsequent operations.
• After that, we call invokevirtual() on the last operands from the stack, which are the constant “sales” and the role parameter. Furthermore, we invoke the equals() method of the String class stored in the stringClass variable to compare the operands. The ClassDesc.of(Z) part says that we expect a boolean as the return type of that method invocation.
• We then call ifeq() passing the notSales label variable. That means the following instructions of the builder will run only if the previous boolean result of invokevirtual() returns true. Otherwise, the program should jump to the notSales binding that we’ll define later.
• Finally, if the condition of ifReq() returns true, we load the baseSalary argument using dload(1). Then, we load the constant 0.35 to the operand stack and use dmul() to multiply the operands stored. Finally, we return the value using dreturn()

That first part covers the first if statement of the method we want to generate. Hence, to generate the second if statement, we can add more method calls to our codeBuilder(), after the dreturn() call:

  .labelBinding(notSales)
  .aload(3)
  .ldc("engineer")
  .invokevirtual(stringClass, "equals", MethodTypeDesc.of(ClassDesc.of("Z"), stringClass))
  .ifeq(notEngineer)
  .dload(1)
  .ldc(0.25)
  .dmul()
  .dreturn()

The labelBinding(notSales) runs if the ifeq(notSales) expression returns false. The other operations are similar to those we previously covered to handle the first if statement.

Finally, we can add the last part to cover the default value return:

  .labelBinding(notEngineer)
  .dload(1)
  .ldc(0.15)
  .dmul()
  .dreturn();

The same thing occurs with labeling branches, but now for the notEngineer label. That last part runs if ifeq(notEngineer) returns false.

Finally, to wrap up our generate() method, we need to define the ClassFile object and write it to a .class file:

var classBuilder = ClassFile.of()
  .build(ClassDesc.of("EmployeeSalaryCalculator"),
    cb - > cb.withMethod("calculateAnnualBonus", MethodTypeDesc.of(CD_double, CD_double, CD_String), 
      AccessFlag.PUBLIC.mask(), 
      calculateAnnualBonusBuilder));

Files.write(Path.of("EmployeeSalaryCalculator.class"), classBuilder);

We’ve used ClassFile.of().build() to instantiate a class file builder, and passed two arguments to it. The first is the class name wrapped inside the ClassDesc.of() call. The second is a ClassBuilder consumer that generates the class with the desired methods. For that, we used withMethod() passing the method name, the method signature, the access flag, and the method code builder defined previously.

Noticeably, we defined the method signature as MethodTypeDesc.of(CD_double, CD_double, CD_String), which means that the method generated returns a double, defined by the first parameter, and receives a double and a String parameter.

Then, we write the byte array stored in the classBuilder variable to a file using the Files writers.

4. Transforming a Class-File Into Another

Now, let’s say we want to copy all the contents of a class file into another. We can do that by using different transformations:

public static void transform() throws IOException {
    var basePath = Files.readAllBytes(Path.of("EmployeeSalaryCalculator.class"));

    CodeTransform codeTransform = ClassFileBuilder::accept;

    MethodTransform methodTransform = MethodTransform.transformingCode(codeTransform);
    ClassTransform classTransform = ClassTransform.transformingMethods(methodTransform);

    ClassFile classFile = ClassFile.of();
    byte[] transformedClass = classFile.transformClass(classFile.parse(basePath), classTransform);
    Files.write(Path.of("TransformedEmployeeSalaryCalculator.class"), transformedClass);
}

In the example above, we first read the class file we created in the previous section, EmployeeSalaryCalculator.

Then, we define a CodeTransform that accepts all CodeElements defined in the original class. Moreover, we create a MethodTransform using the codeTransform and a ClassTransform using the methodTransform. Such a composition makes it easy to generalize and reuse transformers for different purposes.

More customized code and method transforms could be defined using more explicit lambda expressions. For instance, we could define a custom MethodTransform using a lambda expression that only accepts methods with specific names:

MethodTransform methodTransform = (methodBuilder, methodElement) - > {
    if (methodElement.header().name().stringValue().equals("calculateAnnualBonus")) {
        methodBuilder.withCode(codeBuilder - > {
            for (var codeElement: methodElement.code()) {
                codeBuilder.accept(codeElement);
            }
        });
    }
};

In the case above, we first check if the method name is equal to the literal calculateAnnualBonus, using the header() and name() methods. If so, we use the methodBuilder to create a method with the exact instructions from the original class’ methodElement.

5. Conclusion

In this article, we looked at the details of creating classes from scratch and copying content from one class to another using the Class File API.

We examined examples of how to utilize various builders, transformers, and elements to create and transform classes at runtime.

As always, the source code is available over on GitHub.