How can I improve performance of Field.set (perhap using MethodHandles)?

Question

I'm writing some code that calls Field.set and Field.get many many thousands of times. Obviously this is very slow because of the reflection.

I want to see if I can improve performance using MethodHandle in Java 7. So far here's what I have:

Instead of field.set(pojo, value), I'm doing:

private static final Map<Field, MethodHandle> setHandles = new HashMap<>();

MethodHandle mh = setHandles.get(field);
if (mh == null) {
    mh = lookup.unreflectSetter(field);
    setHandles.put(field, mh);
}
mh.invoke(pojo, value);

However, this doesn't seem to perform better than the Field.set call using reflection. Am I doing something wrong here?

I read that using invokeExact could be faster but when I tried using that I got a java.lang.invoke.WrongMethodTypeException.

Has anyone successfully been able to optimize repeated calls to Field.set or Field.get?

Answer 1

2015-06-01: Updated to reflect @JoeC's comment about another case when handles are static. Also updated to latest JMH and re-ran on modern hardware. The conclusion stays almost the same.

Please do proper benchmarking, it is arguably not that hard with JMH. Once you do that, the answer becomes obvious. It can also showcase the proper use of invokeExact (requires target/source 1.7 to compile and run):

@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
@Fork(3)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@State(Scope.Thread)
public class MHOpto {

    private int value = 42;

    private static final Field static_reflective;
    private static final MethodHandle static_unreflect;
    private static final MethodHandle static_mh;

    private static Field reflective;
    private static MethodHandle unreflect;
    private static MethodHandle mh;

    // We would normally use @Setup, but we need to initialize "static final" fields here...
    static {
        try {
            reflective = MHOpto.class.getDeclaredField("value");
            unreflect = MethodHandles.lookup().unreflectGetter(reflective);
            mh = MethodHandles.lookup().findGetter(MHOpto.class, "value", int.class);
            static_reflective = reflective;
            static_unreflect = unreflect;
            static_mh = mh;
        } catch (IllegalAccessException | NoSuchFieldException e) {
            throw new IllegalStateException(e);
        }
    }

    @Benchmark
    public int plain() {
        return value;
    }

    @Benchmark
    public int dynamic_reflect() throws InvocationTargetException, IllegalAccessException {
        return (int) reflective.get(this);
    }

    @Benchmark
    public int dynamic_unreflect_invoke() throws Throwable {
        return (int) unreflect.invoke(this);
    }

    @Benchmark
    public int dynamic_unreflect_invokeExact() throws Throwable {
        return (int) unreflect.invokeExact(this);
    }

    @Benchmark
    public int dynamic_mh_invoke() throws Throwable {
        return (int) mh.invoke(this);
    }

    @Benchmark
    public int dynamic_mh_invokeExact() throws Throwable {
        return (int) mh.invokeExact(this);
    }

    @Benchmark
    public int static_reflect() throws InvocationTargetException, IllegalAccessException {
        return (int) static_reflective.get(this);
    }

    @Benchmark
    public int static_unreflect_invoke() throws Throwable {
        return (int) static_unreflect.invoke(this);
    }

    @Benchmark
    public int static_unreflect_invokeExact() throws Throwable {
        return (int) static_unreflect.invokeExact(this);
    }

    @Benchmark
    public int static_mh_invoke() throws Throwable {
        return (int) static_mh.invoke(this);
    }

    @Benchmark
    public int static_mh_invokeExact() throws Throwable {
        return (int) static_mh.invokeExact(this);
    }

}

On 1x4x2 i7-4790K, JDK 8u40, Linux x86_64 it yields:

Benchmark                             Mode  Cnt  Score   Error  Units
MHOpto.dynamic_mh_invoke              avgt   25  4.393 ± 0.003  ns/op
MHOpto.dynamic_mh_invokeExact         avgt   25  4.394 ± 0.007  ns/op
MHOpto.dynamic_reflect                avgt   25  5.230 ± 0.020  ns/op
MHOpto.dynamic_unreflect_invoke       avgt   25  4.404 ± 0.023  ns/op
MHOpto.dynamic_unreflect_invokeExact  avgt   25  4.397 ± 0.014  ns/op
MHOpto.plain                          avgt   25  1.858 ± 0.002  ns/op
MHOpto.static_mh_invoke               avgt   25  1.862 ± 0.015  ns/op
MHOpto.static_mh_invokeExact          avgt   25  1.859 ± 0.002  ns/op
MHOpto.static_reflect                 avgt   25  4.274 ± 0.011  ns/op
MHOpto.static_unreflect_invoke        avgt   25  1.859 ± 0.002  ns/op
MHOpto.static_unreflect_invokeExact   avgt   25  1.858 ± 0.002  ns/op

...which suggests MH are really much faster than Reflection in this particular case (this is because the access checks against the private field is done at lookup time, and not at the invocation time). dynamic_* cases simulate the case when the MethodHandles and/or Fields are not statically known, e.g. pulled from Map<String, MethodHandle> or something like it. Conversely, static_* cases are those where the invokers are statically known.

Notice the reflective performance is on par with MethodHandles in dynamic_* cases, this is because reflection is heavily optimized further in JDK 8 (because really, you don't need the access check to read your own fields), so the answer may be "just" switching to JDK 8 ;)

static_* cases are even faster, because the MethoHandles.invoke calls are aggressively inlined. This eliminates part of the type checking in MH cases. But, in reflection cases, there are still quick checks present, and therefore, it lags behind.

Answer 2

Update: since some people started a pointless discussion about “how to benchmark” I will emphasize the solution to your problem contained in my answer, now right at the beginning:

You can use invokeExact even in your reflective context where you don’t have the exact type signature by converting the MethodHandle using asType to a handle taking Object as arguments. In environments affected by the performance difference between invoke and invokeExact, using invokeExact on such a converting handle is still way faster than using invoke on a direct method handle.

---

Original answer:

The problem is indeed that you are not using invokeExact. Below is a little benchmark program showing the results of different ways of incrementing an int field. Using invoke instead of invokeExact leads to a performance drop below the speed of Reflection.

You receive the WrongMethodTypeException because the MethodHandle is strongly typed. It expects an exact invocation signature matching type type of the field and owner. But you can use the handle to create a new MethodHandle wrapping the necessary type conversions. Using invokeExact on that handle using a generic signature (i.e. (Object,Object)Object) will be still way more efficient than using invoke with a dynamic type conversion.

The results on my machine using 1.7.0_40 were:

direct        :   27,415ns
reflection    : 1088,462ns
method handle : 7133,221ns
mh invokeExact:   60,928ns
generic mh    :   68,025ns

and using a -server JVM yields to a baffling

direct        :   26,953ns
reflection    :  629,161ns
method handle : 1513,226ns
mh invokeExact:   22,325ns
generic mh    :   43,608ns

I don’t think that it has much real life relevance seeing a MethodHandle being faster than a direct operation but it proves that MethodHandles are not slow on Java7.

And the generic MethodHandle will still outperform Reflection (whilst using invoke does not).

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

public class FieldMethodHandle
{
  public static void main(String[] args)
  {
    final int warmup=1_000_000, iterations=1_000_000;
    for(int i=0; i<warmup; i++)
    {
      incDirect();
      incByReflection();
      incByDirectHandle();
      incByDirectHandleExact();
      incByGeneric();
    }
    long direct=0, refl=0, handle=0, invokeExact=0, genericH=0;
    for(int i=0; i<iterations; i++)
    {
      final long t0=System.nanoTime();
      incDirect();
      final long t1=System.nanoTime();
      incByReflection();
      final long t2=System.nanoTime();
      incByDirectHandle();
      final long t3=System.nanoTime();
      incByDirectHandleExact();
      final long t4=System.nanoTime();
      incByGeneric();
      final long t5=System.nanoTime();
      direct+=t1-t0;
      refl+=t2-t1;
      handle+=t3-t2;
      invokeExact+=t4-t3;
      genericH+=t5-t4;
    }
    final int result = VALUE.value;
    // check (use) the value to avoid over-optimizations
    if(result != (warmup+iterations)*5) throw new AssertionError();
    double r=1D/iterations;
    System.out.printf("%-14s:\t%8.3fns%n", "direct", direct*r);
    System.out.printf("%-14s:\t%8.3fns%n", "reflection", refl*r);
    System.out.printf("%-14s:\t%8.3fns%n", "method handle", handle*r);
    System.out.printf("%-14s:\t%8.3fns%n", "mh invokeExact", invokeExact*r);
    System.out.printf("%-14s:\t%8.3fns%n", "generic mh", genericH*r);
  }
  static class MyValueHolder
  {
    int value;
  }
  static final MyValueHolder VALUE=new MyValueHolder();

  static final MethodHandles.Lookup LOOKUP=MethodHandles.lookup();
  static final MethodHandle DIRECT_GET_MH, DIRECT_SET_MH;
  static final MethodHandle GENERIC_GET_MH, GENERIC_SET_MH;
  static final Field REFLECTION;
  static
  {
    try
    {
      REFLECTION = MyValueHolder.class.getDeclaredField("value");
      DIRECT_GET_MH = LOOKUP.unreflectGetter(REFLECTION);
      DIRECT_SET_MH = LOOKUP.unreflectSetter(REFLECTION);
      GENERIC_GET_MH = DIRECT_GET_MH.asType(DIRECT_GET_MH.type().generic());
      GENERIC_SET_MH = DIRECT_SET_MH.asType(DIRECT_SET_MH.type().generic());
    }
    catch(NoSuchFieldException | IllegalAccessException ex)
    {
      throw new ExceptionInInitializerError(ex);
    }
  }

  static void incDirect()
  {
    VALUE.value++;
  }
  static void incByReflection()
  {
    try
    {
      REFLECTION.setInt(VALUE, REFLECTION.getInt(VALUE)+1);
    }
    catch(IllegalAccessException ex)
    {
      throw new AssertionError(ex);
    }
  }
  static void incByDirectHandle()
  {
    try
    {
      Object target=VALUE;
      Object o=GENERIC_GET_MH.invoke(target);
      o=((Integer)o)+1;
      DIRECT_SET_MH.invoke(target, o);
    }
    catch(Throwable ex)
    {
      throw new AssertionError(ex);
    }
  }
  static void incByDirectHandleExact()
  {
    try
    {
      DIRECT_SET_MH.invokeExact(VALUE, (int)DIRECT_GET_MH.invokeExact(VALUE)+1);
    }
    catch(Throwable ex)
    {
      throw new AssertionError(ex);
    }
  }
  static void incByGeneric()
  {
    try
    {
      Object target=VALUE;
      Object o=GENERIC_GET_MH.invokeExact(target);
      o=((Integer)o)+1;
      o=GENERIC_SET_MH.invokeExact(target, o);
    }
    catch(Throwable ex)
    {
      throw new AssertionError(ex);
    }
  }
}