What is Byte Buddy?

Byte Buddy is a code generation library for creating Java classes during the runtime of a Java application and without the help of a compiler. Other than the code generation utilities that ship with the Java Class Library, Byte Buddy allows the creation of arbitrary classes and is not limited to implementing interfaces for the creation of runtime proxies.

In order to use Byte Buddy, one does not require an understanding of Java byte code or the class file format. In contrast, Byte Buddy’s API aims for code that is concise and easy to understand for everybody. Nevertheless, Byte Buddy remains fully customizable down to the possibility of defining custom byte code. Furthermore, the API was designed to be as non-intrusive as possible and as a result, Byte Buddy does not leave any trace in the classes that were created by it. For this reason, the generated classes can exist without requiring Byte Buddy on the class path. Because of this feature, Byte Buddy’s mascot was chosen to be a ghost.

Byte Buddy is written in Java 6 but supports the generation of classes for any Java version. Byte Buddy is a light-weight library and only depends on the visitor API of the Java byte code parser library ASM which does itself not require any further dependencies.

At first sight, runtime code generation can appear to be some sort of black magic that should be avoided and only few developers write applications that explicitly generate code during their runtime. However, this picture changes when creating libraries that need to interact with arbitrary code and types that are unknown at compile time. In this context, a library implementer must often choose between either requiring a user to implement library-proprietary interfaces or to generate code at runtime when the user’s types becomes first known to the library. Many known libraries such as for example Spring or Hibernate choose the latter approach which is popular among their users under the term of using Plain Old Java Objects. As a result, code generation has become an ubiquitous concept in the Java space. Byte Buddy is an attempt to innovate the runtime creation of Java types in order to provide a better tool set to those relying on code generation.

Hello World

Saying Hello World with Byte Buddy is as easy as it can get. Any creation of a Java class starts with an instance of the ByteBuddy class which represents a configuration for creating new types:

Class<?> dynamicType = new ByteBuddy()
  .subclass(Object.class) 
  .method(named("toString")).intercept(FixedValue.value("Hello World!"))
  .make()
  .load(getClass().getClassLoader(), ClassLoadingStrategy.Default.WRAPPER)
  .getLoaded();
assertThat(dynamicType.newInstance().toString(), is("Hello World!"));

The default ByteBuddy configuration which is used in the above example creatse a Java class in the newest version of the class file format that is understood by the processing Java virtual machine. As hopefully obvious from the example code, the created type will extend the Object class and intercept its toString method which should return a fixed value of Hello World!. The method to be intercepted is identified by a so-called method matcher. In the above example, a predefined method matcher named(String) is used which identifies a method by its exact name. Byte Buddy comes with numerous predefined and well-tested method matchers which are collected in the MethodMatchers class. The creation of custom matchers is however as simple as implementing the (functional) MethodMatcher interface.

For implementing the toString method, the FixedValue class defines a constant return value for the intercepted method. Defining a constant value is only one example of many method interceptors that ship with Byte Buddy. By implementing the Instrumentation interface, a method could however even be defined by custom byte code.

Finally, the described Java class is created and then loaded into the Java virtual machine. For this purpose, a target class loader is required as well as a class loading strategy where we choose a wrapper strategy. The latter creates a new child class loader which wraps the given class loader and only knows about the newly created dynamic type. Eventually, we can convince ourselves of the result by calling the toString method on an instance of the created class and finding the return value to represent the constant value we expected.

A more complex example

Of course, a Hello World example is a too simple use case for evaluating the quality of a code generation library. In reality, a user of such a library wants to perform more complex manipulations such as introducing additional logic to a compiled Java program. Using Byte Buddy, doing so is however not much harder and the following example gives a taste of how method calls can be intercepted.

For this demonstration, we will make up a simple pseudo domain where Account objects can be used for transferring money to a given recipient where the latter is represented by a simple string. Furthermore, we want to express that the direct transfer of money by calling the transfer method is somewhat unsafe which is why we annotate the method with @Unsafe.

@Retention(RetentionPolicy.RUNTIME)
@interface Unsafe { }

@Retention(RetentionPolicy.RUNTIME)
@interface Secured { }

class Account {
  private int amount = 100;
  @Unsafe
  public String transfer(int amount, String recipient) {
    this.amount -= amount;
    return "transferred $" + amount + " to " + recipient;
  }
}

In order to make a transaction safe, we rather want to process it by some Bank. For this purpose, the bank provides an obfuscation but logs the transaction details internally (and to your console). It will then conduct the transaction for the customer. With the help of Byte Buddy, we will now create a subclass of Account that processes all its transfers by using a Bank.

class Bank {
  public static String obfuscate(@Argument(1) String recipient,
                                 @Argument(0) Integer amount,
                                 @Super Account zuper) {
    System.out.println("Transfer " + amount + " to " + recipient);
    return zuper.transfer(amount, recipient.substring(0, 3) + "XXX") + " (obfuscated)";
  }
}

Note the annotations on the Bank's obfuscation method's parameters. The first both arguments are annotated with @Argument(n) which will instruct Byte Buddy to inject the n-th argument of any intercepted method into the annotated parameter. Further, note that the order of the parameters of the Bank's obfuscation method is opposite to the intercepted method in Account. Also note how Byte Buddy is capable of auto-boxing the Integer value. The third parameter of the Bank's obfuscation method is annotated with @Super which instructs Byte Buddy to create a proxy that allows to call the non-intercepted (super) implementations of the extended type.

For the given implementation of a Bank, we can now create a BankAccount using ByteBuddy:

Class<? extends Account> dynamicType = new ByteBuddy()
  .subclass(Account.class)
  .name("BankAccount")
  .method(isAnnotatedBy(Unsafe.class)).intercept(MethodDelegation.to(Bank.class))
  .annotateType(new Secured() {
    @Override
    public Class<? extends Annotation> annotationType() {
      return Secured.class;
    }
  })
  .implement(Serializable.class)
  .make()
  .load(getClass().getClassLoader(), ClassLoadingStrategy.Default.WRAPPER)
  .getLoaded();
assertThat(dynamicType.getName(), is("BankAccount"));
assertThat(dynamicType.isAnnotationPresent(Secured.class), is(true));
assertThat(Serializable.class.isAssignableFrom(dynamicType), is(true));
assertThat(dynamicType.newInstance().transfer(26, "123456"),
    is("transferred $26 to 123XXX (obfuscated)"));

As obvious from the test results, the dynamically generated BankAccount class works as expected. We instructed Byte Buddy to intercept any method call to methods that are annotated by the Unsafe annotation which only concerns the Account's transfer method. We then define to intercept the method call to be delegated by the MethodDelegation instrumentation which will automatically detect the only static method of Bank as a possible delegation target. The documentation of the MethodDelegation class gives details on how a method delegation is discovered and how it can be specified in further detail.

Since all transactions will now be processed by a Bank, we can mark the new class as Secured. We can add this annotation by simply handing over an instance of the annotation to add to the class. Since Java annotations are nothing more than interfaces, this is straight-forward, even though it might appear a little strange at first. Finally, we instruct the Serializable interface to be implemented by the created type.

The created type will be written directly in the Java class file format and in Java byte code and never exist as Java source code or as the source code of another JVM language. However, if you had written a class like the one we just created with Byte Buddy in Java, you would end up with the following Java source code:

@Secured
class BankAccount extends Account implements Serializable {

  private class Proxy extends Account {

    @Override
    public String transfer(int amount, String recipientAccount) {
      return BankAccount.super.transfer(amount, recipientAccount);
    }

    // Omitted overridable methods of java.lang.Object which are also
    // implemented to call the super method implementations of the outer
    // BankAccount instance.
  }

  @Override
  public String transfer(int amount, String recipientAccount) {
    return Bank.obfuscate(recipientAccount, new Integer(amount), new Proxy());
  }
}

You can check out the documentation of the MethodDelegation class for more information. There are plenty of other options for delegation such as delegating to a class or an instance member. And there are other instrumentations that ship with ByteBuddy and were not yet mentioned. One of them allows the implementation of StubMethods. The Exceptional instrumentation allows to throw exceptions. One can conduct a SuperMethodCall or implement a FieldAccessor. Or one can adapt Java Class Library proxies by using an InvocationHandlerAdapter. Just as for the MethodDelegation, the Java documentation is a good place to getting started. Give Byte Buddy a try! You will like it.

Where to go from here?

Byte Buddy is a comprehensive library and we only scratched the surface of Byte Buddy's capabilities. However, Byte Buddy aims for being easy to use by providing a domain-specific language for creating classes. Most runtime code generation can be done by writing readable code and without any knowledge of Java's class file format. If you want to learn more about Byte Buddy, you can find such a tutorial on Byte Buddy's web page. Furthermore, Byte Buddy comes with a detailed in-code documentation and extensive test case coverage which can also serve as example code. When using Byte Buddy, make also sure to read the following information on maintaining a project dependency.

Dependency and API evolution

Byte Buddy is written on top of ASM, a mature and well-tested library for reading and writing compiled Java classes. In order to allow for advanced type manipulations, Byte Buddy is intentionally exposing the ASM API to its users. Of course, the direct use of ASM remains fully optional and most users will most likely never require it. This choice was made such that a user of Byte Buddy is not restrained to its higher-level functionality but can implement custom instrumentations without a fuzz when it is necessary.

However, this imposes one possible problem when relying onto Byte Buddy as a project dependency and making use of the exposed ASM API. The authors of ASM require their users to repackage the ASM dependency into a different name space. This is necessary because one cannot anticipate changes in the Java class file format what can lead to API incompatibilities of future versions of ASM. Because of this, each version of Byte Buddy is distributed in two different packaging formats:

• A no-dependency version that repackages the ASM dependency from its org.objectweb.asm into Byte Buddy's own namespace net.bytebuddy.jar.asm. Doing so, the ASM dependency is also contained within Byte Buddy's jar file. By using this version, you do not need to worry about possible ASM version clashes which might be caused by the use of ASM by both Byte Buddy and other libraries. If you do not plan to use ASM, do not know what ASM is or what this is all about, this is the version you want to use. The artifact ID of this packaging format is byte-buddy.
• A version with an explicit dependency on ASM in its original org.objectweb.asm namespace. This version must only be used for repackaging Byte Buddy and its ASM dependency into your own namespace. Never distribute your application while directly relying on this dependency. Otherwise, your users might experience version conflicts of different ASM versions on their class path. The artifact ID of this packaging format is byte-buddy-dep.

Normally, you would use the first, no-dependency version. However, if you are using Byte Buddy and making use of the exposed ASM API, you must use the second version of Byte Buddy and repackage it into your own name space as suggested. This is in particularly true when you plan to redistribute your code for the use by others. Future versions of Byte Buddy will update their ASM dependency to newer version what will then lead to version clashes between different ASM versions that were repackaged by Byte Buddy, if you have not follow this recommendation! In contrast, the Byte Buddy API itself will only apply version compatible changes.

There exist several tools that allow for an easy automatization of the repacking of dependencies during your build processes. You can for example use the Shade plugin for Maven. With Gradle, a similar tool is the Shadow plugin. Another alternative is jarjar, a library that offers integration as an Ant task.

License and development

Byte Buddy is licensed under the liberal and business-friendly Apache Licence, Version 2.0 and is freely available on this GitHub page. Byte Buddy is further released on Maven Central. The project is built using Maven. From your shell, building the project would look something like this:

git clone https://github.com/raphw/byte-buddy.git
cd byte-buddy
mvn package

On these commands, Byte Buddy is cloned from GitHub and built on your machine. Byte Buddy is currently tested for the OpenJDK versions 6 and 7 and the Oracle JDK versions 7 and 8 using Travis CI. Please use GitHub's issue tracker for reporting bugs. When committing code, please provide test cases that prove the functionality of your features or that demonstrate a bug fix. Furthermore, make sure you are not breaking any existing test cases. If possible, please take the time to write some documentation. For feature requests or general feedback, you can also use the issue tracker or contact us on our mailing list.