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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file, and Oracle licenses the original version of this file under the BSD
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/*
   Copyright 2009-2013 Attila Szegedi

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions are
   met:
   * Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the distribution.
   * Neither the name of the copyright holder nor the names of
     contributors may be used to endorse or promote products derived from
     this software without specific prior written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
   IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
   TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
   PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL COPYRIGHT HOLDER
   BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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   SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
   BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
   WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
   OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
   ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

package jdk.dynalink;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodType;
import java.lang.invoke.MutableCallSite;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Objects;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
import java.util.Set;
import java.util.function.Supplier;
import jdk.dynalink.beans.BeansLinker;
import jdk.dynalink.internal.AccessControlContextFactory;
import jdk.dynalink.linker.GuardedInvocation;
import jdk.dynalink.linker.GuardedInvocationTransformer;
import jdk.dynalink.linker.GuardingDynamicLinker;
import jdk.dynalink.linker.GuardingDynamicLinkerExporter;
import jdk.dynalink.linker.GuardingTypeConverterFactory;
import jdk.dynalink.linker.LinkRequest;
import jdk.dynalink.linker.LinkerServices;
import jdk.dynalink.linker.MethodHandleTransformer;
import jdk.dynalink.linker.MethodTypeConversionStrategy;
import jdk.dynalink.linker.support.CompositeGuardingDynamicLinker;
import jdk.dynalink.linker.support.CompositeTypeBasedGuardingDynamicLinker;
import jdk.dynalink.linker.support.DefaultInternalObjectFilter;
import jdk.dynalink.linker.support.TypeUtilities;

A factory class for creating DynamicLinker objects. Dynamic linkers are the central objects in Dynalink; these are composed of several GuardingDynamicLinker objects and coordinate linking of call sites with them. The usual dynamic linker is a linker composed of all GuardingDynamicLinker objects explicitly pre-created by the user of the factory and configured with setPrioritizedLinkers(List<? extends GuardingDynamicLinker>), as well as any automatically discovered ones, and finally the ones configured with setFallbackLinkers(List<? extends GuardingDynamicLinker>); this last category usually includes BeansLinker. /**
 * A factory class for creating {@link DynamicLinker} objects. Dynamic linkers
 * are the central objects in Dynalink; these are composed of several
 * {@link GuardingDynamicLinker} objects and coordinate linking of call sites
 * with them. The usual dynamic linker is a linker
 * composed of all {@link GuardingDynamicLinker} objects explicitly pre-created
 * by the user of the factory and configured with
 * {@link #setPrioritizedLinkers(List)}, as well as any
 * {@link #setClassLoader(ClassLoader) automatically discovered} ones, and
 * finally the ones configured with {@link #setFallbackLinkers(List)}; this last
 * category usually includes {@link BeansLinker}.
 */
public final class DynamicLinkerFactory {
    private static final AccessControlContext GET_CLASS_LOADER_CONTEXT =
            AccessControlContextFactory.createAccessControlContext("getClassLoader");

    
Default value for unstable relink
threshold. /**
     * Default value for {@link #setUnstableRelinkThreshold(int) unstable relink
     * threshold}.
     */
    private static final int DEFAULT_UNSTABLE_RELINK_THRESHOLD = 8;

    private boolean classLoaderExplicitlySet = false;
    private ClassLoader classLoader;

    private List<? extends GuardingDynamicLinker> prioritizedLinkers;
    private List<? extends GuardingDynamicLinker> fallbackLinkers;
    private boolean syncOnRelink = false;
    private int unstableRelinkThreshold = DEFAULT_UNSTABLE_RELINK_THRESHOLD;
    private GuardedInvocationTransformer prelinkTransformer;
    private MethodTypeConversionStrategy autoConversionStrategy;
    private MethodHandleTransformer internalObjectsFilter;

    private List<ServiceConfigurationError> autoLoadingErrors = Collections.emptyList();

    
Creates a new dynamic linker factory with default configuration. Upon creation, the factory can be configured using various setXxx() methods and used to create one or more dynamic linkers according to its current configuration using createLinker(). /**
     * Creates a new dynamic linker factory with default configuration. Upon
     * creation, the factory can be configured using various {@code setXxx()}
     * methods and used to create one or more dynamic linkers according to its
     * current configuration using {@link #createLinker()}.
     */
    public DynamicLinkerFactory() {
    }

    
Sets the class loader for automatic discovery of available guarding dynamic linkers. GuardingDynamicLinkerExporter implementations available through this class loader will be automatically instantiated using the ServiceLoader mechanism and the linkers they provide will be incorporated into DynamicLinkers that this factory creates. This allows for cross-language interoperability where call sites belonging to this language runtime can be linked by linkers from these automatically discovered runtimes if their native objects are passed to this runtime. If class loader is not set explicitly by invoking this method, then the thread context class loader of the thread invoking createLinker() will be used. If this method is invoked explicitly with null then ServiceLoader.loadInstalled(Class<Object>) will be used to load the linkers. 
Params:
classLoader – the class loader used for the automatic discovery of
available linkers./**
     * Sets the class loader for automatic discovery of available guarding
     * dynamic linkers. {@link GuardingDynamicLinkerExporter} implementations
     * available through this class loader will be automatically instantiated
     * using the {@link ServiceLoader} mechanism and the linkers they provide
     * will be incorporated into {@code DynamicLinker}s that this factory
     * creates. This allows for cross-language interoperability where call sites
     * belonging to this language runtime can be linked by linkers from these
     * automatically discovered runtimes if their native objects are passed to
     * this runtime. If class loader is not set explicitly by invoking this
     * method, then the thread context class loader of the thread invoking
     * {@link #createLinker()} will be used. If this method is invoked
     * explicitly with null then {@link ServiceLoader#loadInstalled(Class)} will
     * be used to load the linkers.
     *
     * @param classLoader the class loader used for the automatic discovery of
     * available linkers.
     */
    public void setClassLoader(final ClassLoader classLoader) {
        this.classLoader = classLoader;
        classLoaderExplicitlySet = true;
    }

    
Sets the prioritized guarding dynamic linkers. Language runtimes using
Dynalink will usually have at least one linker for their own language.
These linkers will be consulted first by the resulting dynamic linker
when it is linking call sites, before any autodiscovered and fallback
linkers. If the factory also autodiscovers a linker class matching one
of the prioritized linkers, the autodiscovered class will be ignored and
the explicit prioritized instance will be used.
Params:
prioritizedLinkers – the list of prioritized linkers. Can be null.
Throws:
NullPointerException – if any of the list elements are null./**
     * Sets the prioritized guarding dynamic linkers. Language runtimes using
     * Dynalink will usually have at least one linker for their own language.
     * These linkers will be consulted first by the resulting dynamic linker
     * when it is linking call sites, before any autodiscovered and fallback
     * linkers. If the factory also autodiscovers a linker class matching one
     * of the prioritized linkers, the autodiscovered class will be ignored and
     * the explicit prioritized instance will be used.
     *
     * @param prioritizedLinkers the list of prioritized linkers. Can be null.
     * @throws NullPointerException if any of the list elements are null.
     */
    public void setPrioritizedLinkers(final List<? extends GuardingDynamicLinker> prioritizedLinkers) {
        this.prioritizedLinkers = copyListRequireNonNullElements(prioritizedLinkers);
    }

    
Sets the prioritized guarding dynamic linkers. Identical to calling setPrioritizedLinkers(List<? extends GuardingDynamicLinker>) with Arrays.asList(prioritizedLinkers). 
Params:
prioritizedLinkers – an array of prioritized linkers. Can be null.
Throws:
NullPointerException – if any of the array elements are null./**
     * Sets the prioritized guarding dynamic linkers. Identical to calling
     * {@link #setPrioritizedLinkers(List)} with
     * {@code Arrays.asList(prioritizedLinkers)}.
     *
     * @param prioritizedLinkers an array of prioritized linkers. Can be null.
     * @throws NullPointerException if any of the array elements are null.
     */
    public void setPrioritizedLinkers(final GuardingDynamicLinker... prioritizedLinkers) {
        setPrioritizedLinkers(prioritizedLinkers == null ? null : Arrays.asList(prioritizedLinkers));
    }

    
Sets a single prioritized linker. Identical to calling setPrioritizedLinkers(List<? extends GuardingDynamicLinker>) with a single-element list. 
Params:
prioritizedLinker – the single prioritized linker. Must not be null.
Throws:
NullPointerException – if null is passed./**
     * Sets a single prioritized linker. Identical to calling
     * {@link #setPrioritizedLinkers(List)} with a single-element list.
     *
     * @param prioritizedLinker the single prioritized linker. Must not be null.
     * @throws NullPointerException if null is passed.
     */
    public void setPrioritizedLinker(final GuardingDynamicLinker prioritizedLinker) {
        this.prioritizedLinkers = Collections.singletonList(Objects.requireNonNull(prioritizedLinker));
    }

    
Sets the fallback guarding dynamic linkers. These linkers will be
consulted last by the resulting dynamic linker when it is linking call
sites, after any autodiscovered and prioritized linkers. If the factory
also autodiscovers a linker class matching one of the fallback linkers,
the autodiscovered class will be ignored and the explicit fallback
instance will be used.
Params:
fallbackLinkers – the list of fallback linkers. Can be empty to indicate the caller wishes to set no fallback linkers. Note that if this method is not invoked explicitly or is passed null, then the factory will create an instance of BeansLinker to serve as the default fallback linker.
Throws:
NullPointerException – if any of the list elements are null./**
     * Sets the fallback guarding dynamic linkers. These linkers will be
     * consulted last by the resulting dynamic linker when it is linking call
     * sites, after any autodiscovered and prioritized linkers. If the factory
     * also autodiscovers a linker class matching one of the fallback linkers,
     * the autodiscovered class will be ignored and the explicit fallback
     * instance will be used.
     *
     * @param fallbackLinkers the list of fallback linkers. Can be empty to
     * indicate the caller wishes to set no fallback linkers. Note that if this
     * method is not invoked explicitly or is passed null, then the factory
     * will create an instance of {@link BeansLinker} to serve as the default
     * fallback linker.
     * @throws NullPointerException if any of the list elements are null.
     */
    public void setFallbackLinkers(final List<? extends GuardingDynamicLinker> fallbackLinkers) {
        this.fallbackLinkers = copyListRequireNonNullElements(fallbackLinkers);
    }

    
Sets the fallback guarding dynamic linkers. Identical to calling setFallbackLinkers(List<? extends GuardingDynamicLinker>) with Arrays.asList(fallbackLinkers). 
Params:
fallbackLinkers – an array of fallback linkers. Can be empty to indicate the caller wishes to set no fallback linkers. Note that if this method is not invoked explicitly or is passed null, then the factory will create an instance of BeansLinker to serve as the default fallback linker.
Throws:
NullPointerException – if any of the array elements are null./**
     * Sets the fallback guarding dynamic linkers. Identical to calling
     * {@link #setFallbackLinkers(List)} with
     * {@code Arrays.asList(fallbackLinkers)}.
     *
     * @param fallbackLinkers an array of fallback linkers. Can be empty to
     * indicate the caller wishes to set no fallback linkers. Note that if this
     * method is not invoked explicitly or is passed null, then the factory
     * will create an instance of {@link BeansLinker} to serve as the default
     * fallback linker.
     * @throws NullPointerException if any of the array elements are null.
     */
    public void setFallbackLinkers(final GuardingDynamicLinker... fallbackLinkers) {
        setFallbackLinkers(fallbackLinkers == null ? null : Arrays.asList(fallbackLinkers));
    }

    
Sets whether the dynamic linker created by this factory will invoke MutableCallSite.syncAll(MutableCallSite[]) after a call site is relinked. Defaults to false. You probably want to set it to true if your runtime supports multithreaded execution of dynamically linked code. 
Params:
syncOnRelink – true for invoking sync on relink, false otherwise./**
     * Sets whether the dynamic linker created by this factory will invoke
     * {@link MutableCallSite#syncAll(MutableCallSite[])} after a call site is
     * relinked. Defaults to false. You probably want to set it to true if your
     * runtime supports multithreaded execution of dynamically linked code.
     * @param syncOnRelink true for invoking sync on relink, false otherwise.
     */
    public void setSyncOnRelink(final boolean syncOnRelink) {
        this.syncOnRelink = syncOnRelink;
    }

    
Sets the unstable relink threshold; the number of times a call site is
relinked after which it will be considered unstable, and subsequent link
requests for it will indicate this. Defaults to 8 when not set explicitly.
Params:
unstableRelinkThreshold – the new threshold. Must not be less than
zero. The value of zero means that call sites will never be considered
unstable.
See Also:
LinkRequest.isCallSiteUnstable()/**
     * Sets the unstable relink threshold; the number of times a call site is
     * relinked after which it will be considered unstable, and subsequent link
     * requests for it will indicate this. Defaults to 8 when not set explicitly.
     * @param unstableRelinkThreshold the new threshold. Must not be less than
     * zero. The value of zero means that call sites will never be considered
     * unstable.
     * @see LinkRequest#isCallSiteUnstable()
     */
    public void setUnstableRelinkThreshold(final int unstableRelinkThreshold) {
        if(unstableRelinkThreshold < 0) {
            throw new IllegalArgumentException("unstableRelinkThreshold < 0");
        }
        this.unstableRelinkThreshold = unstableRelinkThreshold;
    }

    
Set the pre-link transformer. This is a GuardedInvocationTransformer that will get the final chance to modify the guarded invocation after it has been created by a component linker and before the dynamic linker links it into the call site. It is normally used to adapt the return value type of the invocation to the type of the call site. When not set explicitly, a default pre-link transformer will be used that simply calls GuardedInvocation.asType(LinkerServices, MethodType). Customized pre-link transformers are rarely needed; they are mostly used as a building block for implementing advanced techniques such as code deoptimization strategies. 
Params:
prelinkTransformer – the pre-link transformer for the dynamic
linker. Can be null to have the factory use the default transformer./**
     * Set the pre-link transformer. This is a
     * {@link GuardedInvocationTransformer} that will get the final chance to
     * modify the guarded invocation after it has been created by a component
     * linker and before the dynamic linker links it into the call site. It is
     * normally used to adapt the return value type of the invocation to the
     * type of the call site. When not set explicitly, a default pre-link
     * transformer will be used that simply calls
     * {@link GuardedInvocation#asType(LinkerServices, MethodType)}. Customized
     * pre-link transformers are rarely needed; they are mostly used as a
     * building block for implementing advanced techniques such as code
     * deoptimization strategies.
     * @param prelinkTransformer the pre-link transformer for the dynamic
     * linker. Can be null to have the factory use the default transformer.
     */
    public void setPrelinkTransformer(final GuardedInvocationTransformer prelinkTransformer) {
        this.prelinkTransformer = prelinkTransformer;
    }

    
Sets an object representing the conversion strategy for automatic type conversions. After LinkerServices.asType(MethodHandle, MethodType) has applied all custom conversions to a method handle, it still needs to effect method
invocation conversions that can usually be automatically applied as per MethodHandle.asType(MethodType). However, sometimes language runtimes will want to customize even those conversions for their own call sites. A typical example is allowing unboxing of null return values, which is by default prohibited by ordinary MethodHandles.asType(). In this case, a language runtime can install its own custom automatic conversion strategy, that can deal with null values. Note that when the strategy's MethodTypeConversionStrategy.asType(MethodHandle, MethodType) is invoked, the custom language conversions will already have been applied to the method handle, so by design the difference between the handle's current method type and the desired final type will always only be ones that can be subjected to method invocation conversions. The strategy also doesn't need to invoke a final MethodHandle.asType() as that will be done internally as the final step. 
Params:
autoConversionStrategy – the strategy for applying method invocation
conversions for the linker created by this factory. Can be null for no
custom strategy./**
     * Sets an object representing the conversion strategy for automatic type
     * conversions. After
     * {@link LinkerServices#asType(MethodHandle, MethodType)} has applied all
     * custom conversions to a method handle, it still needs to effect
     * {@link TypeUtilities#isMethodInvocationConvertible(Class, Class) method
     * invocation conversions} that can usually be automatically applied as per
     * {@link MethodHandle#asType(MethodType)}. However, sometimes language
     * runtimes will want to customize even those conversions for their own call
     * sites. A typical example is allowing unboxing of null return values,
     * which is by default prohibited by ordinary
     * {@code MethodHandles.asType()}. In this case, a language runtime can
     * install its own custom automatic conversion strategy, that can deal with
     * null values. Note that when the strategy's
     * {@link MethodTypeConversionStrategy#asType(MethodHandle, MethodType)}
     * is invoked, the custom language conversions will already have been
     * applied to the method handle, so by design the difference between the
     * handle's current method type and the desired final type will always only
     * be ones that can be subjected to method invocation conversions. The
     * strategy also doesn't need to invoke a final
     * {@code MethodHandle.asType()} as that will be done internally as the
     * final step.
     * @param autoConversionStrategy the strategy for applying method invocation
     * conversions for the linker created by this factory. Can be null for no
     * custom strategy.
     */
    public void setAutoConversionStrategy(final MethodTypeConversionStrategy autoConversionStrategy) {
        this.autoConversionStrategy = autoConversionStrategy;
    }

    
Sets a method handle transformer that is supposed to act as the implementation of LinkerServices.filterInternalObjects(MethodHandle) for linker services of dynamic linkers created by this factory. Some language runtimes can have internal objects that should not escape their scope. They can add a transformer here that will modify 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. The transformer can also potentially add an unwrapping filter to the return value. DefaultInternalObjectFilter is provided as a convenience class for easily creating such filtering transformers. 
Params:
internalObjectsFilter – a method handle transformer filtering out
internal objects, or null./**
     * Sets a method handle transformer that is supposed to act as the
     * implementation of
     * {@link LinkerServices#filterInternalObjects(MethodHandle)} for linker
     * services of dynamic linkers created by this factory. Some language
     * runtimes can have internal objects that should not escape their scope.
     * They can add a transformer here that will modify 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. The transformer can also
     * potentially add an unwrapping filter to the return value.
     * {@link DefaultInternalObjectFilter} is provided as a convenience class
     * for easily creating such filtering transformers.
     * @param internalObjectsFilter a method handle transformer filtering out
     * internal objects, or null.
     */
    public void setInternalObjectsFilter(final MethodHandleTransformer internalObjectsFilter) {
        this.internalObjectsFilter = internalObjectsFilter;
    }

    
Creates a new dynamic linker based on the current configuration. This method can be invoked more than once to create multiple dynamic linkers. Automatically discovered linkers are newly instantiated on every invocation of this method. It is allowed to change the factory's configuration between invocations. The method is not thread safe. After invocation, callers can invoke getAutoLoadingErrors() to retrieve a list of ServiceConfigurationErrors that occurred while trying to load automatically discovered linkers. These are never thrown from the call to this method as it makes every effort to recover from them and ignore the failing linkers. 
Returns:
the new dynamic Linker/**
     * Creates a new dynamic linker based on the current configuration. This
     * method can be invoked more than once to create multiple dynamic linkers.
     * Automatically discovered linkers are newly instantiated on every
     * invocation of this method. It is allowed to change the factory's
     * configuration between invocations. The method is not thread safe. After
     * invocation, callers can invoke {@link #getAutoLoadingErrors()} to
     * retrieve a list of {@link ServiceConfigurationError}s that occurred while
     * trying to load automatically discovered linkers. These are never thrown
     * from the call to this method as it makes every effort to recover from
     * them and ignore the failing linkers.
     * @return the new dynamic Linker
     */
    public DynamicLinker createLinker() {
        // Treat nulls appropriately
        if(prioritizedLinkers == null) {
            prioritizedLinkers = Collections.emptyList();
        }
        if(fallbackLinkers == null) {
            fallbackLinkers = Collections.singletonList(new BeansLinker());
        }

        // Gather classes of all precreated (prioritized and fallback) linkers.
        // We'll filter out any discovered linkers of the same class.
        final Set<Class<? extends GuardingDynamicLinker>> knownLinkerClasses =
                new HashSet<>();
        addClasses(knownLinkerClasses, prioritizedLinkers);
        addClasses(knownLinkerClasses, fallbackLinkers);

        final List<GuardingDynamicLinker> discovered = discoverAutoLoadLinkers();
        // Now, concatenate ...
        final List<GuardingDynamicLinker> linkers =
                new ArrayList<>(prioritizedLinkers.size() + discovered.size()
                        + fallbackLinkers.size());
        // ... prioritized linkers, ...
        linkers.addAll(prioritizedLinkers);
        // ... filtered discovered linkers, ...
        for(final GuardingDynamicLinker linker: discovered) {
            if(!knownLinkerClasses.contains(linker.getClass())) {
                linkers.add(linker);
            }
        }
        // ... and finally fallback linkers.
        linkers.addAll(fallbackLinkers);
        final List<GuardingDynamicLinker> optimized = CompositeTypeBasedGuardingDynamicLinker.optimize(linkers);
        final GuardingDynamicLinker composite;
        switch(linkers.size()) {
            case 0: {
                composite = (r, s) -> null; // linker that can't link anything
                break;
            }
            case 1: {
                composite = optimized.get(0);
                break;
            }
            default: {
                composite = new CompositeGuardingDynamicLinker(optimized);
                break;
            }
        }

        final List<GuardingTypeConverterFactory> typeConverters = new LinkedList<>();
        for(final GuardingDynamicLinker linker: linkers) {
            if(linker instanceof GuardingTypeConverterFactory) {
                typeConverters.add((GuardingTypeConverterFactory)linker);
            }
        }

        if(prelinkTransformer == null) {
            prelinkTransformer = (inv, request, linkerServices) -> inv.asType(linkerServices, request.getCallSiteDescriptor().getMethodType());
        }

        return new DynamicLinker(new LinkerServicesImpl(new TypeConverterFactory(typeConverters,
                autoConversionStrategy), composite, internalObjectsFilter), prelinkTransformer,
                syncOnRelink, unstableRelinkThreshold);
    }

    
Returns a list of ServiceConfigurationErrors that were encountered while loading automatically discovered linkers during the last invocation of createLinker(). They can be any non-Dynalink specific service configuration issues, as well as some Dynalink-specific errors when an exporter that the factory tried to automatically load: 

did not have the runtime permission named GuardingDynamicLinkerExporter.AUTOLOAD_PERMISSION_NAME in a system with a security manager, or
returned null from Supplier<List<GuardingDynamicLinker>>.get(), or
the list returned from Supplier<List<GuardingDynamicLinker>>.get() had a null element.

Returns:
an immutable list of encountered ServiceConfigurationErrors. Can be empty./**
     * Returns a list of {@link ServiceConfigurationError}s that were
     * encountered while loading automatically discovered linkers during the
     * last invocation of {@link #createLinker()}. They can be any non-Dynalink
     * specific service configuration issues, as well as some Dynalink-specific
     * errors when an exporter that the factory tried to automatically load:
     * <ul>
     * <li>did not have the runtime permission named
     * {@link GuardingDynamicLinkerExporter#AUTOLOAD_PERMISSION_NAME} in a
     * system with a security manager, or</li>
     * <li>returned null from {@link GuardingDynamicLinkerExporter#get()}, or</li>
     * <li>the list returned from {@link GuardingDynamicLinkerExporter#get()}
     * had a null element.</li>
     * </ul>
     * @return an immutable list of encountered
     * {@link ServiceConfigurationError}s. Can be empty.
     */
    public List<ServiceConfigurationError> getAutoLoadingErrors() {
        return Collections.unmodifiableList(autoLoadingErrors);
    }

    private List<GuardingDynamicLinker> discoverAutoLoadLinkers() {
        autoLoadingErrors = new LinkedList<>();
        final ClassLoader effectiveClassLoader = classLoaderExplicitlySet ? classLoader : getThreadContextClassLoader();
        final List<GuardingDynamicLinker> discovered = new LinkedList<>();
        try {
            final ServiceLoader<GuardingDynamicLinkerExporter> linkerLoader =
                    AccessController.doPrivileged((PrivilegedAction<ServiceLoader<GuardingDynamicLinkerExporter>>)()-> {
                        if (effectiveClassLoader == null) {
                            return ServiceLoader.loadInstalled(GuardingDynamicLinkerExporter.class);
                        }
                        return ServiceLoader.load(GuardingDynamicLinkerExporter.class, effectiveClassLoader);
                    });

            for(final Iterator<GuardingDynamicLinkerExporter> it = linkerLoader.iterator(); it.hasNext();) {
                try {
                    final GuardingDynamicLinkerExporter autoLoader = it.next();
                    try {
                        discovered.addAll(requireNonNullElements(
                                Objects.requireNonNull(autoLoader.get(),
                                        ()->(autoLoader.getClass().getName() + " returned null from get()")),
                                ()->(autoLoader.getClass().getName() + " returned a list with at least one null element")));
                    } catch (final ServiceConfigurationError|VirtualMachineError e) {
                        // Don't wrap a SCE in another SCE. Also, don't ignore
                        // any VME (e.g. StackOverflowError or OutOfMemoryError).
                        throw e;
                    } catch (final Throwable t) {
                        throw new ServiceConfigurationError(t.getMessage(), t);
                    }
                } catch (final ServiceConfigurationError e) {
                    // Catch SCE with an individual exporter, carry on with it.hasNext().
                    autoLoadingErrors.add(e);
                }
            }
        } catch (final ServiceConfigurationError e) {
            // Catch a top-level SCE; one either in ServiceLoader.load(),
            // ServiceLoader.iterator(), or Iterator.hasNext().
            autoLoadingErrors.add(e);
        }
        return discovered;
    }

    private static ClassLoader getThreadContextClassLoader() {
        return AccessController.doPrivileged(
            (PrivilegedAction<ClassLoader>) () -> Thread.currentThread().getContextClassLoader(),
            GET_CLASS_LOADER_CONTEXT);
    }

    private static void addClasses(final Set<Class<? extends GuardingDynamicLinker>> knownLinkerClasses,
            final List<? extends GuardingDynamicLinker> linkers) {
        for(final GuardingDynamicLinker linker: linkers) {
            knownLinkerClasses.add(linker.getClass());
        }
    }

    private static <T> List<T> copyListRequireNonNullElements(final List<T> list) {
        if (list == null) {
            return null;
        }
        return new ArrayList<>(requireNonNullElements(list, ()->"List has at least one null element"));
    }

    private static <T> List<T> requireNonNullElements(final List<T> list, final Supplier<String> msgSupplier) {
        for(final T t: list) {
            Objects.requireNonNull(t, msgSupplier);
        }
        return list;
    }

}