Avoiding sun.misc.Unsafe and embracing modules in Java libraries: missing links

[Henri Tremblay]

Hi Rafael,

I don't like the idea to have to explicitly load a module to create mocks.

Because Unsafe.allocateInstance is of course used to create mock but also
proxies. For instance, Spring uses it extensively through Objenesis. And
different frameworks doing serialization.

Also, It means Gradle and Maven would need to be updated to load the module
automatically in test context. And all IDE. A lot of annoyance for not much
benefit since everybody is currently happy.

I've been advocating that if creating a class without calling a constructor
is useless in multiple contexts, it should be easily available through the
API. I don't know exactly where to put it but it should be

   - Available to all (java.base)
   - Protected by the security manager

For completeness, there are 4 ways to create a class without calling a
constructor right now that I'm aware of:

   - Unsafe.allocateInstance
   - sun.reflect.ReflectionFactory.newConstructorForSerialization (my
   favorite because it's the fastest one)
   - Generate an extending class forgetting to call the super constructor
   (so it's not exactly that same class that is instantiated). It requires
   -Xverify:none
   - Generate a class extending MagicAccessorImpl that will then
   instantiates the wanted class but calling the wrong constructor

Regards,
Henri



On 1 April 2018 at 17:02, Rafael Winterhalter <rafael.wth at gmail.com> wrote:

> Hello,
>
> I am the/a maintainer of the libraries Byte Buddy, cglib and Mockito with
> countless dependencies upstream and I wanted to give a summary of adopting
> the JPMS and migrating away from sun.misc.Unsafe.
>
> 1. Java agents cannot define auxiliary classes.
>
> Byte Buddy does support the JPMS fully, however, it still relies on
> sun.misc.Unsafe::defineClass for its Java agent API and currently breaks on
> Java 11 as this method was removed in a recent EA build. The reason for
> using Unsafe is that many instrumentations need to define auxiliary classes
> to aid an instrumentation similar to javac which sometimes needs to define
> anonymous classes or even synthetic classes. For example, if a Java agent
> wants to register an event listener to some framework, such listeners often
> declare multiple methods what makes it impossible to fullfil the listener
> contract using a lambda expression. Instead, one typically injects an
> additional class into the same package as the instrumented class. In this
> case, it is not possible to use MethodHandles.Lookup::defineClass as the
> class file transformer does not necessarily have private access to the
> lookup of the instrumented class.
>
> The current workarounds are:
>
> a) Open the package jdk.internal.misc to gain access to this package's
> Unsafe class. This can be done via Instrumentation::redefineModule.
> b) Open the java.lang package to access ClassLoader via reflection.
> c) Open the java.lang package to access the internal lookup with global
> access rights.
>
> Of these solutions only (b) relies on standard API and is guaranteed to
> function in the future but the solution still feels hacky and does not work
> for instrumentations of classes on the bootstrap loader. Opening packages
> also implies a risk of being applied carelessly since opening the package
> to the agent's module most likely opens the package to the unnamed module
> of the system class loader what invites to breaches of the JPMS
> encapsulation by code that does not ship with the agent.
>
> To offer a better solution, I would like to suggest one of the following:
>
> a) Add a method defineClass(ClassLoader, byte[], ProtectionDomain) to the
> Instrumentation interface that works similar to Unsafe::defineClass. This
> would provide a very simple migration path. Since agents have access to
> jdk.internal.misc, adding this method does not add any capabilities to the
> agent, it merley avoids using internal API that might change.
> b) Supply a MethodHandles.Lookup instance to the
> ClassFileTransformer::transform API where the instance represents the
> instrumented class's access rights. This does however carry the risk of
> invoking the lookupClass method which would either load the instrumented
> class prematurely causing a circularity error or return an unexpected value
> such as null. Since the lookup API generally relies on loaded types, there
> are probably other problems such as invoking Lookup::privateLookupIn before
> all involved types are loaded.
>
> For the sake of simlicity and since easy migration paths make a quick
> adoption easier, I would suggestion solution (a), also in the light that
> quick and dirty migrations might still choose option (b) to save time and
> also since (b) might cause problems when types are not yet loaded.
>
> 2. Java proxies cannot invoke default methods of proxied interfaces
>
> The Java proxy API does not currently allow the invocation of an overridden
> default method since
> the InvocationHandler API only supplies an instance of
> java.lang.reflection.Method. In Java 8, it was always possible to get hold
> of method handle of any method of the proxied interface and to specialize
> the handle on the interface type to invoke the default implementation. With
> the JPMS, even if an interface type is exported, this same type might not
> be opened to another module. This implies that if an InvocationHandler is
> defined by this module to which the interface is exported, this module's
> InvocationHandler cannot resolve a specialized method handle to a default
> method of the proxied interface. As a matter of fact, such a call is
> impossible in this scenario whereas the same call is possible if the proxy
> is implemented manually at compile time.
>
> As a solution, I suggest to provide an argument to the InvocationHandler
> that represents a lookup instance with the access rights of the proxy
> class. Using this lookup, the specialized method handles could be resolved.
>
> 3. Mocking and serialization libraries still require
> Unsafe::allocateInstance.
>
> For Mockito, it is required to instantiate any class without invoking a
> constructor with potential side-effects. This is of course a grose
> violation of Java's general contract for class instantiation but this
> feature is very useful.
>
> Using a Java agent, it is already possible to emulate this feature without
> internal API by instrumenting all constructors of all classes in the
> hierarchy of a mocked class by transforming all constructors into the
> following pseudo-code:
>
> SomeConstructor(Void arg) {
>   if (MockitoBootHelper.THREAD_LOCAL.isMockInstantiatingMode()) {
>     super(null); // any constructor of the super class with default values
> for all arguments
>   } else {
>     // original constructor code...
>   }
> }
>
> Before instantiating a mock, the thread local value that is held by a
> bootstrap-loader injected class is set to true such that a side-effect free
> construction is achieved.
>
> This is of course too expensive and has negative effects on performance due
> to additional branching and JIT-byte code limits such that one would rather
> open jdk.internal.misc to access the Unsafe instantiation mechanism if a
> Java agent is already available.
>
> However, mocking and serialization libraries are not typically loaded as a
> Java agent. Also, I think that the actual requirements are different. My
> suggestion here is:
>
> a) For serialization libraries, I think that adding
> MethodHandles.Lookup::newInstance(Class<? extends Serializable>) with
> standard deserialization mechanics would be sufficient.
> b) For mocking libraries, this does not suffice as mocks can be of any
> class. I understand that this breaks encapsulation but for unit tests, I
> argue that the benefit of using these libraries outweights the benefit of
> full encapsulation within a unit test.
>
> As Mockito is typically run within a build which uses a JDK, we could
> attach to the current VM using the attachment API. Since Java 9, this is no
> longer possible to attach to the same VM but instead we start a helper JVM
> process that applies the attachment indirectly. Unfortunately, this is a
> rather costly operation what is especially problematic when running a
> single unit test. (The difference to this approach over Unsafe is about
> half a second on average.)
>
> To overcome this, I would like to suggest to:
>
> a) Add a method Instrumentation::allocateInstance(Class). Java agents can
> already emulate this privilege as described above, this is therefore merely
> a convenience.
> b) Add a module jdk.test to JDK 11 with a class
> JavaTest::getInstrumentation that returns an instrumentation instance for
> the current JVM. This module should not be resolved by default but only
> when requiring it explicitly similar to the EE modules after Java 9.
>
> I think this solution carries two benefits:
>
> a) Test libraries like Mockito can only be used in a testing scope. We
> experience regularly that Mockito is used in production environments. The
> library is not meant for that and we warn and document that this is not
> intended use but people regularly ignore this directive. By requiring this
> module, this form of abuse would no longer be possible and the JVM could
> even give a meaningful error message if this use was intended.
> b) The Instrumentation instance can be used for other meaningful testing
> approaches. For example, Mockito offers an inline mock maker where the
> mocking logic is inlined into a method body rather then creating a
> subclass. This approach mainly targets final classes which have become more
> common especially since the Kotlin language became popular. To supply this
> alternative mock maker, Mockito attempts attaching an agent to the current
> VM (directly or indirectly, depending on the VM's version) which suffers
> the additional costs for attaching an agent that I mentioned before.
>
> Thanks for reading this and I hope that you can consider these, as of
> today, very practiced use cases. The JPMS migration has gone quite well, I
> find. With these outstanding problems, JDK 11 could be the first release
> where a majority of Java programs would no longer need to rely on internal
> API.
>
> Best regards, Rafael
>