dynalink/linker/LinkerServices.java

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/*
   Copyright 2009-2013 Attila Szegedi

   Licensed under both the Apache License, Version 2.0 (the "Apache License")
   and the BSD License (the "BSD License"), with licensee being free to
   choose either of the two at their discretion.

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package jdk.dynalink.linker;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.util.function.Supplier;
import jdk.dynalink.DynamicLinker;
import jdk.dynalink.DynamicLinkerFactory;
import jdk.dynalink.SecureLookupSupplier;
import jdk.dynalink.linker.ConversionComparator.Comparison;
import jdk.dynalink.linker.support.TypeUtilities;

/**
 * Interface for services provided to {@link GuardingDynamicLinker} instances by
 * the {@link DynamicLinker} that owns them.
 */
public interface LinkerServices {
    /**
     * Similar to {@link MethodHandle#asType(MethodType)} except it also hooks
     * in method handles produced by all available
     * {@link GuardingTypeConverterFactory} implementations, providing for
     * language-specific type coercing of parameters. It will apply
     * {@link MethodHandle#asType(MethodType)} for all primitive-to-primitive,
     * wrapper-to-primitive, primitive-to-wrapper conversions as well as for all
     * upcasts. For all other conversions, it'll insert
     * {@link MethodHandles#filterArguments(MethodHandle, int, MethodHandle...)}
     * with composite filters provided by {@link GuardingTypeConverterFactory}
     * implementations.
     *
     * @param handle target method handle
     * @param fromType the types of source arguments
     * @return a method handle that is a suitable combination of
     * {@link MethodHandle#asType(MethodType)},
     * {@link MethodHandles#filterArguments(MethodHandle, int, MethodHandle...)},
     * and {@link MethodHandles#filterReturnValue(MethodHandle, MethodHandle)}
     * with {@link GuardingTypeConverterFactory}-produced type converters as
     * filters.
     */
    public MethodHandle asType(MethodHandle handle, MethodType fromType);

    /**
     * Similar to {@link #asType(MethodHandle, MethodType)} except it treats
     * return value type conversion specially. It only converts the return type
     * of the method handle when it can be done using a conversion that loses
     * neither precision nor magnitude, otherwise it leaves it unchanged. These
     * are the only return value conversions that should be performed by
     * individual language-specific linkers, and
     * {@link DynamicLinkerFactory#setPrelinkTransformer(GuardedInvocationTransformer)
     * pre-link transformer of the dynamic linker} should implement the strategy
     * for dealing with potentially lossy return type conversions in a manner
     * specific to the language runtime where the call site is located.
     *
     * @param handle target method handle
     * @param fromType the types of source arguments
     * @return a method handle that is a suitable combination of
     * {@link MethodHandle#asType(MethodType)}, and
     * {@link MethodHandles#filterArguments(MethodHandle, int, MethodHandle...)}
     * with {@link GuardingTypeConverterFactory}-produced type converters as filters.
     */
    public default MethodHandle asTypeLosslessReturn(final MethodHandle handle, final MethodType fromType) {
        final Class<?> handleReturnType = handle.type().returnType();
        return asType(handle, TypeUtilities.isConvertibleWithoutLoss(handleReturnType, fromType.returnType()) ?
                fromType : fromType.changeReturnType(handleReturnType));
    }

    /**
     * Given a source and target type, returns a method handle that converts
     * between them. Never returns null; in worst case it will return an
     * identity conversion (that might fail for some values at runtime). You
     * rarely need to use this method directly and should mostly rely on
     * {@link #asType(MethodHandle, MethodType)} instead. This method is needed
     * when you need to reuse existing type conversion machinery outside the
     * context of processing a link request.
     * @param sourceType the type to convert from
     * @param targetType the type to convert to
     * @return a method handle performing the conversion.
     */
    public MethodHandle getTypeConverter(Class<?> sourceType, Class<?> targetType);

    /**
     * Returns true if there might exist a conversion between the requested
     * types (either an automatic JVM conversion, or one provided by any
     * available {@link GuardingTypeConverterFactory}), or false if there
     * definitely does not exist a conversion between the requested types. Note
     * that returning true does not guarantee that the conversion will succeed
     * at runtime for all values (especially if the "from" or "to" types are
     * sufficiently generic), but returning false guarantees that it would fail.
     *
     * @param from the source type for the conversion
     * @param to the target type for the conversion
     * @return true if there can be a conversion, false if there can not.
     */
    public boolean canConvert(Class<?> from, Class<?> to);

    /**
     * Creates a guarded invocation delegating back to the {@link DynamicLinker}
     * that exposes this linker services object. The dynamic linker will then
     * itself delegate the linking to all of its managed
     * {@link GuardingDynamicLinker}s including potentially this one if no
     * linker responds earlier, so beware of infinite recursion. You'll
     * typically craft the link request so that it will be different than the
     * one you are currently trying to link.
     *
     * @param linkRequest a request for linking the invocation
     * @return a guarded invocation linked by some of the guarding dynamic
     * linkers managed by the top-level dynamic linker. Can be null if no
     * available linker is able to link the invocation. You will typically use
     * the elements of the returned invocation to compose your own invocation.
     * @throws Exception in case the top-level linker throws an exception
     */
    public GuardedInvocation getGuardedInvocation(LinkRequest linkRequest) throws Exception;

    /**
     * Determines which of the two type conversions from a source type to the
     * two target types is preferred. This is used for dynamic overloaded method
     * resolution. If the source type is convertible to exactly one target type
     * with a method invocation conversion, it is chosen, otherwise available
     * {@link ConversionComparator}s are consulted.
     * @param sourceType the source type.
     * @param targetType1 one potential target type
     * @param targetType2 another potential target type.
     * @return one of Comparison constants that establish which &ndash; if any
     * &ndash; of the target types is preferable for the conversion.
     */
    public Comparison compareConversion(Class<?> sourceType, Class<?> targetType1, Class<?> targetType2);

    /**
     * Modifies the method handle so that any parameters that can receive
     * potentially internal language runtime objects will have a filter added on
     * them to prevent them from escaping, potentially by wrapping them. It can
     * also potentially add an unwrapping filter to the return value. Basically
     * transforms the method handle using the transformer configured by
     * {@link DynamicLinkerFactory#setInternalObjectsFilter(MethodHandleTransformer)}.
     * @param target the target method handle
     * @return a method handle with parameters and/or return type potentially
     * filtered for wrapping and unwrapping.
     */
    public MethodHandle filterInternalObjects(final MethodHandle target);

    /**
     * Executes an operation within the context of a particular
     * {@code MethodHandles.Lookup} lookup object. Normally, methods on
     * {@code LinkerServices} are invoked as part of the linking mechanism in
     * which case Dynalink internally maintains a per-thread current lookup
     * (the one belonging to the descriptor of the call site being linked). This
     * lookup can be retrieved by any {@link GuardingTypeConverterFactory}
     * involved in linking if it needs to generate lookup-sensitive converters.
     * However, linker services' methods can be invoked outside the linking
     * process too when implementing invocation-time dispatch schemes, invoking
     * conversions at runtime, etc. If it becomes necessary to use any type
     * converter in this situation, and it needs a lookup, it will normally only
     * get {@link MethodHandles#publicLookup()} as the thread is not engaged in
     * a linking operation. If there is a way to meaningfully associate the
     * operation to the context of some caller class, consider performing it
     * within an invocation of this method and passing a full-strength lookup
     * for that class, as it will associate that lookup with the current thread
     * for the duration of the operation. Note that since you are passing a
     * {@link SecureLookupSupplier}, any invoked type converter factories will
     * still need to hold the necessary runtime permission to be able to get the
     * lookup should they need it.
     * @param <T> the type of the return value provided by the passed-in supplier.
     * @param operation the operation to execute in context of the specified lookup.
     * @param lookupSupplier secure supplier of the lookup
     * @return the return value of the action
     * @throws NullPointerException if either action or lookupSupplier are null.
     * @see GuardingTypeConverterFactory#convertToType(Class, Class, Supplier)
     */
    public <T> T getWithLookup(final Supplier<T> operation, final SecureLookupSupplier lookupSupplier);
}