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JMockit is a Java toolkit for automated developer testing. It contains mocking/faking APIs and a code coverage tool, supporting both JUnit and TestNG. The mocking APIs allow all kinds of Java code, without testability restrictions, to be tested in isolation from selected dependencies.
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/*
 * Copyright (c) 2006 JMockit developers
 * This file is subject to the terms of the MIT license (see LICENSE.txt).
 */
package mockit.internal.startup;

import java.lang.instrument.*;
import javax.annotation.*;

import mockit.internal.*;
import mockit.internal.expectations.transformation.*;
import mockit.internal.state.*;
import static mockit.internal.startup.ClassLoadingBridgeFields.createSyntheticFieldsInJREClassToHoldClassLoadingBridges;

/**
 * This is the "agent class" that initializes the JMockit "Java agent". It is not intended for use in client code.
 * Instead, the JVM needs to be initialized with -javaagent:<properPathTo>/jmockit-1-x.jar.
 *
 * @see #premain(String, Instrumentation)
 */
public final class Startup
{
   public static boolean initializing;
   @Nullable private static Instrumentation instrumentation;

   private Startup() {}

   /**
    * This method must only be called by the JVM, to provide the instrumentation object.
    * In order for this to occur, the JVM must be started with "-javaagent:jmockit-1.x.jar" as a command line parameter
    * (assuming the jar file is in the current directory).
    * 
    * It is also possible to load user-specified fakes at this time, by having set the "fakes" system property.
    *
    * @param agentArgs coverage configuration parameters, if any
    * @param inst      the instrumentation service provided by the JVM
    */
   public static void premain(@Nullable String agentArgs, @Nonnull Instrumentation inst) {
      createSyntheticFieldsInJREClassToHoldClassLoadingBridges(inst);
      instrumentation = inst;
      initialize(inst);
   }

   private static void initialize(@Nonnull Instrumentation inst) {
      inst.addTransformer(CachedClassfiles.INSTANCE, true);
      applyStartupFakes(inst);
      inst.addTransformer(new ExpectationsTransformer());
   }

   private static void applyStartupFakes(@Nonnull Instrumentation inst) {
      initializing = true;
      try { JMockitInitialization.initialize(inst); } finally { initializing = false; }
   }

   @Nonnull @SuppressWarnings("ConstantConditions")
   public static Instrumentation instrumentation() { return instrumentation; }

   public static void verifyInitialization() {
      if (instrumentation == null) {
         throw new IllegalStateException(
            "JMockit didn't get initialized; please check the -javaagent JVM initialization parameter was used");
      }
   }

   @SuppressWarnings("ConstantConditions")
   public static void retransformClass(@Nonnull Class aClass) {
      try { instrumentation.retransformClasses(aClass); } catch (UnmodifiableClassException ignore) {}
   }

   public static void redefineMethods(@Nonnull ClassIdentification classToRedefine, @Nonnull byte[] modifiedClassfile) {
      Class loadedClass = classToRedefine.getLoadedClass();
      redefineMethods(loadedClass, modifiedClassfile);
   }

   public static void redefineMethods(@Nonnull Class classToRedefine, @Nonnull byte[] modifiedClassfile) {
      redefineMethods(new ClassDefinition(classToRedefine, modifiedClassfile));
   }

   public static void redefineMethods(@Nonnull ClassDefinition... classDefs) {
      try {
         //noinspection ConstantConditions
         instrumentation.redefineClasses(classDefs);
      }
      catch (ClassNotFoundException | UnmodifiableClassException e) {
         throw new RuntimeException(e); // should never happen
      }
      catch (InternalError ignore) {
         // If a class to be redefined hasn't been loaded yet, the JVM may get a NoClassDefFoundError during
         // redefinition. Unfortunately, it then throws a plain InternalError instead.
         for (ClassDefinition classDef : classDefs) {
            detectMissingDependenciesIfAny(classDef.getDefinitionClass());
         }

         // If the above didn't throw upon detecting a NoClassDefFoundError, then ignore the original error and
         // continue, in order to prevent secondary failures.
      }
   }

   private static void detectMissingDependenciesIfAny(@Nonnull Class mockedClass) {
      try {
         Class.forName(mockedClass.getName(), false, mockedClass.getClassLoader());
      }
      catch (NoClassDefFoundError e) {
         throw new RuntimeException("Unable to mock " + mockedClass + " due to a missing dependency", e);
      }
      catch (ClassNotFoundException ignore) {
         // Shouldn't happen since the mocked class would already have been found in the classpath.
      }
   }

   @Nullable
   public static Class getClassIfLoaded(@Nonnull String classDescOrName) {
      String className = classDescOrName.replace('/', '.');
      @SuppressWarnings("ConstantConditions") Class[] loadedClasses = instrumentation.getAllLoadedClasses();

      for (Class aClass : loadedClasses) {
         if (aClass.getName().equals(className)) {
            return aClass;
         }
      }

      return null;
   }
}