🧪 Unit Test 001: CAFEBABE

📘 Preface

Every .class file in Java begins with the same hexadecimal value: CAFEBABE. This is known as the magic number, and it's used by the JVM to verify that a file is indeed a valid Java class file.

🔬 Test Case

public class T_001_CafeBabe {

  @Test
  public void test() throws IOException {
    Class clazz = this.getClass();
    String className = clazz.getName();
    String classAsPath = className.replace('.', '/') + ".class";
    try (var stream = clazz.getClassLoader().getResourceAsStream(classAsPath)) {
      var builder = new StringBuilder();
      for (byte b : stream.readNBytes(4)) {
        builder.append(String.format("%02X", b));
      }
      Assertions.assertEquals("CAFEBABE", builder.toString());
    }
  }
}

🔎 Why This Matters

This is a simple but powerful illustration of how the JVM protects itself from corrupt or non-Java files. If the magic number is incorrect, the JVM will throw a ClassFormatError and refuse to load the class. It also shows the quirkiness and satire of the JVM engineers.