/core/src/com/google/inject/internal/Errors.java

/**
 * Copyright (C) 2006 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.inject.internal;

import com.google.common.base.Equivalence;
import com.google.common.base.Objects;
import com.google.common.base.Throwables;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Ordering;
import com.google.inject.ConfigurationException;
import com.google.inject.CreationException;
import com.google.inject.Guice;
import com.google.inject.Key;
import com.google.inject.MembersInjector;
import com.google.inject.Provider;
import com.google.inject.Provides;
import com.google.inject.ProvisionException;
import com.google.inject.Scope;
import com.google.inject.TypeLiteral;
import com.google.inject.internal.util.Classes;
import com.google.inject.internal.util.SourceProvider;
import com.google.inject.internal.util.StackTraceElements;
import com.google.inject.spi.Dependency;
import com.google.inject.spi.ElementSource;
import com.google.inject.spi.InjectionListener;
import com.google.inject.spi.InjectionPoint;
import com.google.inject.spi.Message;
import com.google.inject.spi.ScopeBinding;
import com.google.inject.spi.TypeConverterBinding;
import com.google.inject.spi.TypeListenerBinding;

import java.io.Serializable;
import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Member;
import java.lang.reflect.Method;
import java.lang.reflect.Type;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Formatter;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * A collection of error messages. If this type is passed as a method parameter, the method is
 * considered to have executed successfully only if new errors were not added to this collection.
 *
 * <p>Errors can be chained to provide additional context. To add context, call {@link #withSource}
 * to create a new Errors instance that contains additional context. All messages added to the
 * returned instance will contain full context.
 *
 * <p>To avoid messages with redundant context, {@link #withSource} should be added sparingly. A
 * good rule of thumb is to assume a method's caller has already specified enough context to
 * identify that method. When calling a method that's defined in a different context, call that
 * method with an errors object that includes its context.
 *
 * @author jessewilson@google.com (Jesse Wilson)
 */
public final class Errors implements Serializable {

  private static final Logger logger = Logger.getLogger(Guice.class.getName());

  private static final Set<Dependency<?>> warnedDependencies =
      Collections.newSetFromMap(new ConcurrentHashMap<Dependency<?>, Boolean>());


  /**
   * The root errors object. Used to access the list of error messages.
   */
  private final Errors root;

  /**
   * The parent errors object. Used to obtain the chain of source objects.
   */
  private final Errors parent;

  /**
   * The leaf source for errors added here.
   */
  private final Object source;

  /**
   * null unless (root == this) and error messages exist. Never an empty list.
   */
  private List<Message> errors; // lazy, use getErrorsForAdd()

  public Errors() {
    this.root = this;
    this.parent = null;
    this.source = SourceProvider.UNKNOWN_SOURCE;
  }

  public Errors(Object source) {
    this.root = this;
    this.parent = null;
    this.source = source;
  }

  private Errors(Errors parent, Object source) {
    this.root = parent.root;
    this.parent = parent;
    this.source = source;
  }

  /**
   * Returns an instance that uses {@code source} as a reference point for newly added errors.
   */
  public Errors withSource(Object source) {
    return source == this.source || source == SourceProvider.UNKNOWN_SOURCE
        ? this
        : new Errors(this, source);
  }

  /**
   * We use a fairly generic error message here. The motivation is to share the
   * same message for both bind time errors:
   * <pre><code>Guice.createInjector(new AbstractModule() {
   *   public void configure() {
   *     bind(Runnable.class);
   *   }
   * }</code></pre>
   * ...and at provide-time errors:
   * <pre><code>Guice.createInjector().getInstance(Runnable.class);</code></pre>
   * Otherwise we need to know who's calling when resolving a just-in-time
   * binding, which makes things unnecessarily complex.
   */
  public Errors missingImplementation(Key key) {
    return addMessage("No implementation for %s was bound.", key);
  }

  public Errors jitDisabled(Key key) {
    return addMessage("Explicit bindings are required and %s is not explicitly bound.", key);
  }

  public Errors jitDisabledInParent(Key<?> key) {
    return addMessage(
        "Explicit bindings are required and %s would be bound in a parent injector.%n"
        + "Please add an explicit binding for it, either in the child or the parent.",
        key);
  }

  public Errors atInjectRequired(Class clazz) {
    return addMessage(
        "Explicit @Inject annotations are required on constructors,"
        + " but %s has no constructors annotated with @Inject.",
        clazz);
  }

  public Errors converterReturnedNull(String stringValue, Object source,
      TypeLiteral<?> type, TypeConverterBinding typeConverterBinding) {
    return addMessage("Received null converting '%s' (bound at %s) to %s%n"
        + " using %s.",
        stringValue, convert(source), type, typeConverterBinding);
  }

  public Errors conversionTypeError(String stringValue, Object source, TypeLiteral<?> type,
      TypeConverterBinding typeConverterBinding, Object converted) {
    return addMessage("Type mismatch converting '%s' (bound at %s) to %s%n"
        + " using %s.%n"
        + " Converter returned %s.",
        stringValue, convert(source), type, typeConverterBinding, converted);
  }

  public Errors conversionError(String stringValue, Object source,
      TypeLiteral<?> type, TypeConverterBinding typeConverterBinding, RuntimeException cause) {
    return errorInUserCode(cause, "Error converting '%s' (bound at %s) to %s%n"
        + " using %s.%n"
        + " Reason: %s",
        stringValue, convert(source), type, typeConverterBinding, cause);
  }

  public Errors ambiguousTypeConversion(String stringValue, Object source, TypeLiteral<?> type,
      TypeConverterBinding a, TypeConverterBinding b) {
    return addMessage("Multiple converters can convert '%s' (bound at %s) to %s:%n"
        + " %s and%n"
        + " %s.%n"
        + " Please adjust your type converter configuration to avoid overlapping matches.",
        stringValue, convert(source), type, a, b);
  }

  public Errors bindingToProvider() {
    return addMessage("Binding to Provider is not allowed.");
  }

  public Errors subtypeNotProvided(Class<? extends javax.inject.Provider<?>> providerType,
      Class<?> type) {
    return addMessage("%s doesn't provide instances of %s.", providerType, type);
  }

  public Errors notASubtype(Class<?> implementationType, Class<?> type) {
    return addMessage("%s doesn't extend %s.", implementationType, type);
  }

  public Errors recursiveImplementationType() {
    return addMessage("@ImplementedBy points to the same class it annotates.");
  }

  public Errors recursiveProviderType() {
    return addMessage("@ProvidedBy points to the same class it annotates.");
  }

  public Errors missingRuntimeRetention(Class<? extends Annotation> annotation) {
    return addMessage(format("Please annotate %s with @Retention(RUNTIME).", annotation));
  }

  public Errors missingScopeAnnotation(Class<? extends Annotation> annotation) {
    return addMessage(format("Please annotate %s with @ScopeAnnotation.", annotation));
  }

  public Errors optionalConstructor(Constructor constructor) {
    return addMessage("%s is annotated @Inject(optional=true), "
        + "but constructors cannot be optional.", constructor);
  }

  public Errors cannotBindToGuiceType(String simpleName) {
    return addMessage("Binding to core guice framework type is not allowed: %s.", simpleName);
  }

  public Errors scopeNotFound(Class<? extends Annotation> scopeAnnotation) {
    return addMessage("No scope is bound to %s.", scopeAnnotation);
  }

  public Errors scopeAnnotationOnAbstractType(
      Class<? extends Annotation> scopeAnnotation, Class<?> type, Object source) {
    return addMessage("%s is annotated with %s, but scope annotations are not supported "
        + "for abstract types.%n Bound at %s.", type, scopeAnnotation, convert(source));
  }

  public Errors misplacedBindingAnnotation(Member member, Annotation bindingAnnotation) {
    return addMessage("%s is annotated with %s, but binding annotations should be applied "
        + "to its parameters instead.", member, bindingAnnotation);
  }

  private static final String CONSTRUCTOR_RULES =
      "Classes must have either one (and only one) constructor "
          + "annotated with @Inject or a zero-argument constructor that is not private.";

  public Errors missingConstructor(Class<?> implementation) {
    return addMessage("Could not find a suitable constructor in %s. " + CONSTRUCTOR_RULES,
        implementation);
  }

  public Errors tooManyConstructors(Class<?> implementation) {
    return addMessage("%s has more than one constructor annotated with @Inject. "
        + CONSTRUCTOR_RULES, implementation);
  }

  public Errors constructorNotDefinedByType(Constructor<?> constructor, TypeLiteral<?> type) {
    return addMessage("%s does not define %s", type, constructor);
  }

  public Errors duplicateScopes(ScopeBinding existing,
      Class<? extends Annotation> annotationType, Scope scope) {
    return addMessage("Scope %s is already bound to %s at %s.%n Cannot bind %s.",
        existing.getScope(), annotationType, existing.getSource(), scope);
  }

  public Errors voidProviderMethod() {
    return addMessage("Provider methods must return a value. Do not return void.");
  }

  public Errors missingConstantValues() {
    return addMessage("Missing constant value. Please call to(...).");
  }

  public Errors cannotInjectInnerClass(Class<?> type) {
    return addMessage("Injecting into inner classes is not supported.  "
        + "Please use a 'static' class (top-level or nested) instead of %s.", type);
  }

  public Errors duplicateBindingAnnotations(Member member,
      Class<? extends Annotation> a, Class<? extends Annotation> b) {
    return addMessage("%s has more than one annotation annotated with @BindingAnnotation: "
        + "%s and %s", member, a, b);
  }

  public Errors staticInjectionOnInterface(Class<?> clazz) {
    return addMessage("%s is an interface, but interfaces have no static injection points.", clazz);
  }

  public Errors cannotInjectFinalField(Field field) {
    return addMessage("Injected field %s cannot be final.", field);
  }

  public Errors cannotInjectAbstractMethod(Method method) {
    return addMessage("Injected method %s cannot be abstract.", method);
  }

  public Errors cannotInjectNonVoidMethod(Method method) {
    return addMessage("Injected method %s must return void.", method);
  }

  public Errors cannotInjectMethodWithTypeParameters(Method method) {
    return addMessage("Injected method %s cannot declare type parameters of its own.", method);
  }

  public Errors duplicateScopeAnnotations(
      Class<? extends Annotation> a, Class<? extends Annotation> b) {
    return addMessage("More than one scope annotation was found: %s and %s.", a, b);
  }

  public Errors recursiveBinding() {
    return addMessage("Binding points to itself.");
  }

  public Errors bindingAlreadySet(Key<?> key, Object source) {
    return addMessage("A binding to %s was already configured at %s.", key, convert(source));
  }

  public Errors jitBindingAlreadySet(Key<?> key) {
    return addMessage("A just-in-time binding to %s was already configured on a parent injector.", key);
  }

  public Errors childBindingAlreadySet(Key<?> key, Set<Object> sources) {
    Formatter allSources = new Formatter();
    for (Object source : sources) {
      if (source == null) {
        allSources.format("%n    (bound by a just-in-time binding)");
      } else {
        allSources.format("%n    bound at %s", source);
      }
    }
    Errors errors = addMessage(
        "Unable to create binding for %s."
      + " It was already configured on one or more child injectors or private modules"
      + "%s%n"
      + "  If it was in a PrivateModule, did you forget to expose the binding?",
        key, allSources.out());
    return errors;
  }

  public Errors errorCheckingDuplicateBinding(Key<?> key, Object source, Throwable t) {
    return addMessage(
        "A binding to %s was already configured at %s and an error was thrown "
      + "while checking duplicate bindings.  Error: %s",
        key, convert(source), t);
  }

  public Errors errorInjectingMethod(Throwable cause) {
    return errorInUserCode(cause, "Error injecting method, %s", cause);
  }

  public Errors errorNotifyingTypeListener(TypeListenerBinding listener,
      TypeLiteral<?> type, Throwable cause) {
    return errorInUserCode(cause,
        "Error notifying TypeListener %s (bound at %s) of %s.%n"
        + " Reason: %s",
        listener.getListener(), convert(listener.getSource()), type, cause);
  }

  public Errors errorInjectingConstructor(Throwable cause) {
    return errorInUserCode(cause, "Error injecting constructor, %s", cause);
  }

  public Errors errorInProvider(RuntimeException runtimeException) {
    Throwable unwrapped = unwrap(runtimeException);
    return errorInUserCode(unwrapped, "Error in custom provider, %s", unwrapped);
  }

  public Errors errorInUserInjector(
      MembersInjector<?> listener, TypeLiteral<?> type, RuntimeException cause) {
    return errorInUserCode(cause, "Error injecting %s using %s.%n"
        + " Reason: %s", type, listener, cause);
  }

  public Errors errorNotifyingInjectionListener(
      InjectionListener<?> listener, TypeLiteral<?> type, RuntimeException cause) {
    return errorInUserCode(cause, "Error notifying InjectionListener %s of %s.%n"
        + " Reason: %s", listener, type, cause);
  }

  public Errors exposedButNotBound(Key<?> key) {
    return addMessage("Could not expose() %s, it must be explicitly bound.", key);
  }

  public Errors keyNotFullySpecified(TypeLiteral<?> typeLiteral) {
    return addMessage("%s cannot be used as a key; It is not fully specified.", typeLiteral);
  }

  public Errors errorEnhancingClass(Class<?> clazz, Throwable cause) {
    return errorInUserCode(cause, "Unable to method intercept: %s", clazz);
  }

  public static Collection<Message> getMessagesFromThrowable(Throwable throwable) {
    if (throwable instanceof ProvisionException) {
      return ((ProvisionException) throwable).getErrorMessages();
    } else if (throwable instanceof ConfigurationException) {
      return ((ConfigurationException) throwable).getErrorMessages();
    } else if (throwable instanceof CreationException) {
      return ((CreationException) throwable).getErrorMessages();
    } else {
      return ImmutableSet.of();
    }
  }

  public Errors errorInUserCode(Throwable cause, String messageFormat, Object... arguments) {
    Collection<Message> messages = getMessagesFromThrowable(cause);

    if (!messages.isEmpty()) {
      return merge(messages);
    } else {
      return addMessage(cause, messageFormat, arguments);
    }
  }

  private Throwable unwrap(RuntimeException runtimeException) {
   if(runtimeException instanceof Exceptions.UnhandledCheckedUserException) {
     return runtimeException.getCause();
   } else {
     return runtimeException;
   }
  }

  public Errors cannotInjectRawProvider() {
    return addMessage("Cannot inject a Provider that has no type parameter");
  }

  public Errors cannotInjectRawMembersInjector() {
    return addMessage("Cannot inject a MembersInjector that has no type parameter");
  }

  public Errors cannotInjectTypeLiteralOf(Type unsupportedType) {
    return addMessage("Cannot inject a TypeLiteral of %s", unsupportedType);
  }

  public Errors cannotInjectRawTypeLiteral() {
    return addMessage("Cannot inject a TypeLiteral that has no type parameter");
  }

  public Errors cannotProxyClass(Class<?> expectedType) {
    return addMessage(
        "Tried proxying %s to support a circular dependency, but it is not an interface.",
        expectedType);
  }

  public Errors circularDependenciesDisabled(Class<?> expectedType) {
    return addMessage(
        "Found a circular dependency involving %s, and circular dependencies are disabled.",
        expectedType);
  }

  public void throwCreationExceptionIfErrorsExist() {
    if (!hasErrors()) {
      return;
    }

    throw new CreationException(getMessages());
  }

  public void throwConfigurationExceptionIfErrorsExist() {
    if (!hasErrors()) {
      return;
    }

    throw new ConfigurationException(getMessages());
  }

  public void throwProvisionExceptionIfErrorsExist() {
    if (!hasErrors()) {
      return;
    }

    throw new ProvisionException(getMessages());
  }

  private Message merge(Message message) {
    List<Object> sources = Lists.newArrayList();
    sources.addAll(getSources());
    sources.addAll(message.getSources());
    return new Message(sources, message.getMessage(), message.getCause());
  }

  public Errors merge(Collection<Message> messages) {
    for (Message message : messages) {
      addMessage(merge(message));
    }
    return this;
  }

  public Errors merge(Errors moreErrors) {
    if (moreErrors.root == root || moreErrors.root.errors == null) {
      return this;
    }

    merge(moreErrors.root.errors);
    return this;
  }

  public List<Object> getSources() {
    List<Object> sources = Lists.newArrayList();
    for (Errors e = this; e != null; e = e.parent) {
      if (e.source != SourceProvider.UNKNOWN_SOURCE) {
        sources.add(0, e.source);
      }
    }
    return sources;
  }

  public void throwIfNewErrors(int expectedSize) throws ErrorsException {
    if (size() == expectedSize) {
      return;
    }

    throw toException();
  }

  public ErrorsException toException() {
    return new ErrorsException(this);
  }

  public boolean hasErrors() {
    return root.errors != null;
  }

  public Errors addMessage(String messageFormat, Object... arguments) {
    return addMessage(null, messageFormat, arguments);
  }

  private Errors addMessage(Throwable cause, String messageFormat, Object... arguments) {
    String message = format(messageFormat, arguments);
    addMessage(new Message(getSources(), message, cause));
    return this;
  }

  public Errors addMessage(Message message) {
    if (root.errors == null) {
      root.errors = Lists.newArrayList();
    }
    root.errors.add(message);
    return this;
  }

  public static String format(String messageFormat, Object... arguments) {
    for (int i = 0; i < arguments.length; i++) {
      arguments[i] = Errors.convert(arguments[i]);
    }
    return String.format(messageFormat, arguments);
  }

  public List<Message> getMessages() {
    if (root.errors == null) {
      return ImmutableList.of();
    }

    return new Ordering<Message>() {
      @Override
      public int compare(Message a, Message b) {
        return a.getSource().compareTo(b.getSource());
      }
    }.sortedCopy(root.errors);
  }

  /** Returns the formatted message for an exception with the specified messages. */
  public static String format(String heading, Collection<Message> errorMessages) {
    Formatter fmt = new Formatter().format(heading).format(":%n%n");
    int index = 1;
    boolean displayCauses = getOnlyCause(errorMessages) == null;

    Map<Equivalence.Wrapper<Throwable>, Integer> causes = Maps.newHashMap();
    for (Message errorMessage : errorMessages) {
      int thisIdx = index++;
      fmt.format("%s) %s%n", thisIdx, errorMessage.getMessage());

      List<Object> dependencies = errorMessage.getSources();
      for (int i = dependencies.size() - 1; i >= 0; i--) {
        Object source = dependencies.get(i);
        formatSource(fmt, source);
      }

      Throwable cause = errorMessage.getCause();
      if (displayCauses && cause != null) {
        Equivalence.Wrapper<Throwable> causeEquivalence = ThrowableEquivalence.INSTANCE.wrap(cause);
        if (!causes.containsKey(causeEquivalence)) {
          causes.put(causeEquivalence, thisIdx);
          fmt.format("Caused by: %s", Throwables.getStackTraceAsString(cause));
        } else {
          int causeIdx = causes.get(causeEquivalence);
          fmt.format("Caused by: %s (same stack trace as error #%s)",
              cause.getClass().getName(), causeIdx);
        }
      }

      fmt.format("%n");
    }

    if (errorMessages.size() == 1) {
      fmt.format("1 error");
    } else {
      fmt.format("%s errors", errorMessages.size());
    }

    return fmt.toString();
  }

  /**
   * Returns {@code value} if it is non-null allowed to be null. Otherwise a message is added and
   * an {@code ErrorsException} is thrown.
   */
  public <T> T checkForNull(T value, Object source, Dependency<?> dependency)
      throws ErrorsException {
    if (value != null || dependency.isNullable() ) {
      return value;
    }

    // Hack to allow null parameters to @Provides methods, for backwards compatibility.
    if (dependency.getInjectionPoint().getMember() instanceof Method) {
      Method annotated = (Method) dependency.getInjectionPoint().getMember();
      if (annotated.isAnnotationPresent(Provides.class)) {
        switch (InternalFlags.getNullableProvidesOption()) {
          case ERROR:
            break; // break out & let the below exception happen
          case IGNORE:
            return value; // user doesn't care about injecting nulls to non-@Nullables.
          case WARN:
            // Warn only once, otherwise we spam logs too much.
            if (!warnedDependencies.add(dependency)) {
              return value;
            }
            logger.log(Level.WARNING,
                "Guice injected null into parameter {0} of {1} (a {2}), please mark it @Nullable."
                    + " Use -Dguice_check_nullable_provides_params=ERROR to turn this into an"
                    + " error.",
                new Object[] {
                    dependency.getParameterIndex(),
                    convert(dependency.getInjectionPoint().getMember()),
                    convert(dependency.getKey())});
            return null; // log & exit.
        }
      }
    }

    int parameterIndex = dependency.getParameterIndex();
    String parameterName = (parameterIndex != -1)
        ? "parameter " + parameterIndex + " of "
        : "";
    addMessage("null returned by binding at %s%n but %s%s is not @Nullable",
        source, parameterName, dependency.getInjectionPoint().getMember());

    throw toException();
  }

  /**
   * Returns the cause throwable if there is exactly one cause in {@code messages}. If there are
   * zero or multiple messages with causes, null is returned.
   */
  public static Throwable getOnlyCause(Collection<Message> messages) {
    Throwable onlyCause = null;
    for (Message message : messages) {
      Throwable messageCause = message.getCause();
      if (messageCause == null) {
        continue;
      }

      if (onlyCause != null
          && !ThrowableEquivalence.INSTANCE.equivalent(onlyCause, messageCause)) {
        return null;
      }

      onlyCause = messageCause;
    }

    return onlyCause;
  }

  public int size() {
    return root.errors == null ? 0 : root.errors.size();
  }

  private static abstract class Converter<T> {

    final Class<T> type;

    Converter(Class<T> type) {
      this.type = type;
    }

    boolean appliesTo(Object o) {
      return o != null && type.isAssignableFrom(o.getClass());
    }

    String convert(Object o) {
      return toString(type.cast(o));
    }

    abstract String toString(T t);
  }

  private static final Collection<Converter<?>> converters = ImmutableList.of(
      new Converter<Class>(Class.class) {
        @Override public String toString(Class c) {
          return c.getName();
        }
      },
      new Converter<Member>(Member.class) {
        @Override public String toString(Member member) {
          return Classes.toString(member);
        }
      },
      new Converter<Key>(Key.class) {
        @Override public String toString(Key key) {
          if (key.getAnnotationType() != null) {
            return key.getTypeLiteral() + " annotated with "
                + (key.getAnnotation() != null ? key.getAnnotation() : key.getAnnotationType());
          } else {
            return key.getTypeLiteral().toString();
          }
        }
      });

  public static Object convert(Object o) {
    ElementSource source = null;
    if (o instanceof ElementSource) {
      source = (ElementSource)o;
      o = source.getDeclaringSource();
    }
    return convert(o, source);
  }

  public static Object convert(Object o, ElementSource source) {
    for (Converter<?> converter : converters) {
      if (converter.appliesTo(o)) {
        return appendModules(converter.convert(o), source);
      }
    }
    return appendModules(o, source);
  }

  private static Object appendModules(Object source, ElementSource elementSource) {
    String modules = moduleSourceString(elementSource);
    if (modules.length() == 0) {
      return source;
    } else {
      return source + modules;
    }
  }

  private static String moduleSourceString(ElementSource elementSource) {
    // if we only have one module (or don't know what they are), then don't bother
    // reporting it, because the source already is going to report exactly that module.
    if (elementSource == null) {
      return "";
    }
    List<String> modules = Lists.newArrayList(elementSource.getModuleClassNames());
    // Insert any original element sources w/ module info into the path.
    while(elementSource.getOriginalElementSource() != null) {
      elementSource = elementSource.getOriginalElementSource();
      modules.addAll(0, elementSource.getModuleClassNames());
    }
    if (modules.size() <= 1) {
      return "";
    }

    // Ideally we'd do:
    //    return Joiner.on(" -> ")
    //        .appendTo(new StringBuilder(" (via modules: "), Lists.reverse(modules))
    //        .append(")").toString();
    // ... but for some reason we can't find Lists.reverse, so do it the boring way.
    StringBuilder builder = new StringBuilder(" (via modules: ");
    for (int i = modules.size() - 1; i >= 0; i--) {
      builder.append(modules.get(i));
      if (i != 0) {
        builder.append(" -> ");
      }
    }
    builder.append(")");
    return builder.toString();
  }

  public static void formatSource(Formatter formatter, Object source) {
    ElementSource elementSource = null;
    if (source instanceof ElementSource) {
      elementSource = (ElementSource)source;
      source = elementSource.getDeclaringSource();
    }
    formatSource(formatter, source, elementSource);
  }

  public static void formatSource(Formatter formatter, Object source, ElementSource elementSource) {
    String modules = moduleSourceString(elementSource);
    if (source instanceof Dependency) {
      Dependency<?> dependency = (Dependency<?>) source;
      InjectionPoint injectionPoint = dependency.getInjectionPoint();
      if (injectionPoint != null) {
        formatInjectionPoint(formatter, dependency, injectionPoint, elementSource);
      } else {
        formatSource(formatter, dependency.getKey(), elementSource);
      }

    } else if (source instanceof InjectionPoint) {
      formatInjectionPoint(formatter, null, (InjectionPoint) source, elementSource);

    } else if (source instanceof Class) {
      formatter.format("  at %s%s%n", StackTraceElements.forType((Class<?>) source), modules);

    } else if (source instanceof Member) {
      formatter.format("  at %s%s%n", StackTraceElements.forMember((Member) source), modules);

    } else if (source instanceof TypeLiteral) {
      formatter.format("  while locating %s%s%n", source, modules);

    } else if (source instanceof Key) {
      Key<?> key = (Key<?>) source;
      formatter.format("  while locating %s%n", convert(key, elementSource));

    } else if (source instanceof Thread) {
      formatter.format("  in thread %s%n", source);

    } else {
      formatter.format("  at %s%s%n", source, modules);
    }
  }

  public static void formatInjectionPoint(Formatter formatter, Dependency<?> dependency,
      InjectionPoint injectionPoint, ElementSource elementSource) {
    Member member = injectionPoint.getMember();
    Class<? extends Member> memberType = Classes.memberType(member);

    if (memberType == Field.class) {
      dependency = injectionPoint.getDependencies().get(0);
      formatter.format("  while locating %s%n", convert(dependency.getKey(), elementSource));
      formatter.format("    for field at %s%n", StackTraceElements.forMember(member));

    } else if (dependency != null) {
      formatter.format("  while locating %s%n", convert(dependency.getKey(), elementSource));
      formatter.format("    for parameter %s at %s%n",
          dependency.getParameterIndex(), StackTraceElements.forMember(member));

    } else {
      formatSource(formatter, injectionPoint.getMember());
    }
  }

  static class ThrowableEquivalence extends Equivalence<Throwable> {
    static final ThrowableEquivalence INSTANCE = new ThrowableEquivalence();

    @Override protected boolean doEquivalent(Throwable a, Throwable b) {
      return a.getClass().equals(b.getClass())
          && Objects.equal(a.getMessage(), b.getMessage())
          && Arrays.equals(a.getStackTrace(), b.getStackTrace())
          && equivalent(a.getCause(), b.getCause());
    }

    @Override protected int doHash(Throwable t) {
      return Objects.hashCode(t.getClass().hashCode(), t.getMessage(), hash(t.getCause()));
    }
  }
}