java.lang.reflect.Proxy and checked exceptions

On-the-Fly Proxy

Say on have an interface Foo with several (hundreds) methods. Is it possible to implement an interface on-the-fly? Without having an implementation code? Yes. It is possible. The standard possibility is java.lang.reflect.Proxy. The newProxyInstance method helps to create an on-the-fly implementation. One provides an interceptor object that is called for every method invocation on the interface implementation instance.

Besides the standard Proxy API, there are libraries, that do the same thing, for example, Byte Buddy or CGLIB.

In this post, we will use the standard JRE API - java.lang.reflect.Proxy

Proxy and Checked Exceptions

Let’s consider the following code in Java:

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

interface Foo {
  void bar();
}

public static void main(String[] args) {
  final Foo proxy = (Foo)Proxy.newProxyInstance(Foo.class.getClassLoader(), new Class[]{Foo.class}, new InvocationHandler() {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      throw new Exception("fail");
    }
  });
  //what is the exception?
  proxy.bar();
}

The code is trivial. We have the interface Foo, and we implement it via Proxy#newProxyInstance. The implementation of the Proxy instance throws an exception of type Exception. Will we have the exception of type Exception as a result?

Running the Example

Let’s execute the example and see what we have:

Exception in thread "main" java.lang.reflect.UndeclaredThrowableException
	at $Proxy0.bar(Unknown Source)
	at ProxyJava.main(ProxyJava.java:22)
Caused by: java.lang.Exception: fail
	at ProxyJava$1.invoke(ProxyJava.java:17)
	... 2 more

The answer is NO. We have java.lang.reflect.UndeclaredThrowableException exception.

Checked Exceptions in Java

As we all know, Java has checked exceptions. It means one declares what exceptions are possibly thrown from a method. The main class of all exceptions is java.lang.Throwable.

In Java language, we use throws to indicate that a method may throw an exception. For example, throws IOException.

There are two specific sub-classes of Throwable, which does not require to be declared by the throws keyword - java.lang.Error and java.lang.RuntimeException. All sub-classes of those two types are free to throw without declaration.

Proxy and UndeclaredThrowableException

The UndeclaredThrowableException is the specific exception type that is used in the create a proxy implementation of an interface to preserve checked exceptions in Java. As we see from the Javadoc, the exception is used to wrap any checked exceptions that are not declared with the throws block in the interface declaration.

Proxy and JVM Languages

JVM ecosystem is huge. There are many languages for the JVM, including Kotlin, Groovy, Scala and so on, that does not have checked exceptions.

Checked exceptions are checked by the compiler, on the JVM bytecode level, there is no difference between exceptions at all.

It is quite easy to get UndeclaredThrowableException at some unexpected places if mixing such languages with java.lang.reflect.Proxy!

For example, in Kotlin:

import java.lang.reflect.Proxy

internal interface Foo {
  fun bar()
}

fun main(args: Array<String>) {
  val proxy = Proxy.newProxyInstance(
    Foo::class.java.classLoader,
    arrayOf<Class<*>>(Foo::class.java)
  ) { _, _, _ ->
    throw Exception("fail")
  } as Foo

  proxy.bar()
}

The same code reads correctly but does not work. It is allowed in Kotlin to throw Exception from a method (because exceptions are not checked), but it will not work via the java.lang.reflect.Proxy. We will have the following execution result

Exception in thread "main" java.lang.reflect.UndeclaredThrowableException
	at com.sun.proxy.$Proxy0.bar(Unknown Source)
	at ProxyKTKt.main(ProxyKT.kt:15)
Caused by: java.lang.Exception: fail
	at ProxyKTKt$main$proxy$1.invoke(ProxyKT.kt:12)
	at ProxyKTKt$main$proxy$1.invoke(ProxyKT.kt)
	... 2 more

Fixing the UndeclaredThrowableException

To avoid the UndeclaredThrowableException one need to declare the exceptions explicitly with throws block. That solves the problem in Java example above. Similarly, it solves the problem in the Kotlin snippet too: we add the @Throws(Exception::class) annotation on the bar function.

One may have a look at the implementation of the Proxy#newProxyInstance in the sources of JVM. It turns out it is not possible to disable that logic in the implementation. One is not allowed to breach Java’s checked exceptions with Proxy#newProxyInstance.

There are two ways. One is to declare throws for all interfaces that are used with Proxy#newProxyInstance. Of course, it is too easy to forget doing in languages without checked exceptions. Tests may help.

An alternative could be to implement or use another variant of the Proxy#newProxyInstance, that does not do the check. Let me know in the comments if you’d like to learn more, how exactly the Proxy#newProxyInstance or similar proxies are implemented.