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Feign Core
There is a newer version: 9.7.0
package feign;

import feign.InvocationHandlerFactory.MethodHandler;
import org.jvnet.animal_sniffer.IgnoreJRERequirement;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles.Lookup;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * Handles default methods by directly invoking the default method code on the interface.
 * The bindTo method must be called on the result before invoke is called.
 */
@IgnoreJRERequirement
final class DefaultMethodHandler implements MethodHandler {
  // Uses Java 7 MethodHandle based reflection.  As default methods will only exist when
  // run on a Java 8 JVM this will not affect use on legacy JVMs.
  // When Feign upgrades to Java 7, remove the @IgnoreJRERequirement annotation.
  private final MethodHandle unboundHandle;

  // handle is effectively final after bindTo has been called.
  private MethodHandle handle;

  public DefaultMethodHandler(Method defaultMethod) {
    try {
      Class declaringClass = defaultMethod.getDeclaringClass();
      Field field = Lookup.class.getDeclaredField("IMPL_LOOKUP");
      field.setAccessible(true);
      Lookup lookup = (Lookup) field.get(null);

      this.unboundHandle = lookup.unreflectSpecial(defaultMethod, declaringClass);
    } catch (NoSuchFieldException ex) {
      throw new IllegalStateException(ex);
    } catch (IllegalAccessException ex) {
      throw new IllegalStateException(ex);
    }
  }

  /**
   * Bind this handler to a proxy object.  After bound, DefaultMethodHandler#invoke will act as if it was called
   * on the proxy object.  Must be called once and only once for a given instance of DefaultMethodHandler
   */
  public void bindTo(Object proxy) {
    if(handle != null) {
      throw new IllegalStateException("Attempted to rebind a default method handler that was already bound");
    }
    handle = unboundHandle.bindTo(proxy);
  }

  /**
   * Invoke this method.  DefaultMethodHandler#bindTo must be called before the first
   * time invoke is called.
   */
  @Override
  public Object invoke(Object[] argv) throws Throwable {
    if(handle == null) {
      throw new IllegalStateException("Default method handler invoked before proxy has been bound.");
    }
    return handle.invokeWithArguments(argv);
  }
}