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GraphqL Java
There is a newer version: 230521-nf-execution
package graphql.schema;


import graphql.Assert;
import graphql.PublicApi;
import graphql.TrivialDataFetcher;

import java.util.function.Function;

/**
 * This is the default data fetcher used in graphql-java.  It will examine
 * maps and POJO java beans for values that match the desired name, typically the field name
 * or it will use a provided function to obtain values.
 * maps and POJO java beans for values that match the desired name.
 * 
 * It uses the following strategies
 * 

 * If the source is null, return null
 * If the source is a Map, return map.get(propertyName)
 * If a function is provided, it is used
 * Find a public JavaBean getter method named `propertyName`
 * Find any getter method named `propertyName` and call method.setAccessible(true)
 * Find a public field named `propertyName`
 * Find any field named `propertyName` and call field.setAccessible(true)
 * If this cant find anything, then null is returned
 * 
 * 
 * You can write your own data fetchers to get data from some other backing system
 * if you need highly customised behaviour.
 *
 * @see graphql.schema.DataFetcher
 */
@PublicApi
public class PropertyDataFetcher implements DataFetcher, TrivialDataFetcher {

    private final String propertyName;
    private final Function function;

    /**
     * This constructor will use the property name and examine the {@link DataFetchingEnvironment#getSource()}
     * object for a getter method or field with that name.
     *
     * @param propertyName the name of the property to retrieve
     */
    public PropertyDataFetcher(String propertyName) {
        this.propertyName = Assert.assertNotNull(propertyName);
        this.function = null;
    }

    @SuppressWarnings("unchecked")
    private  PropertyDataFetcher(Function function) {
        this.function = (Function) Assert.assertNotNull(function);
        this.propertyName = null;
    }

    /**
     * Returns a data fetcher that will use the property name to examine the {@link DataFetchingEnvironment#getSource()} object
     * for a getter method or field with that name, or if its a map, it will look up a value using
     * property name as a key.
     * 

     * For example :
     * 
     * {@code
     *
     *      DataFetcher functionDataFetcher = fetching("pojoPropertyName");
     *
     * }
     * 
     *
     * @param propertyName the name of the property to retrieve
     * @param           the type of result
     * @return a new PropertyDataFetcher using the provided function as its source of values
     */
    public static  PropertyDataFetcher fetching(String propertyName) {
        return new PropertyDataFetcher<>(propertyName);
    }

    /**
     * Returns a data fetcher that will present the {@link DataFetchingEnvironment#getSource()} object to the supplied
     * function to obtain a value, which allows you to use Java 8 method references say obtain values in a
     * more type safe way.
     * 
     * For example :
     * 
     * {@code
     *
     *      DataFetcher functionDataFetcher = fetching(Thing::getId);
     *
     * }
     * 
     *
     * @param function the function to use to obtain a value from the source object
     * @param       the type of the source object
     * @param       the type of result
     * @return a new PropertyDataFetcher using the provided function as its source of values
     */
    public static  PropertyDataFetcher fetching(Function function) {
        return new PropertyDataFetcher<>(function);
    }

    /**
     * @return the property that this is fetching for
     */
    public String getPropertyName() {
        return propertyName;
    }

    @SuppressWarnings("unchecked")
    @Override
    public T get(DataFetchingEnvironment environment) {
        Object source = environment.getSource();
        if (source == null) {
            return null;
        }

        if (function != null) {
            return (T) function.apply(source);
        }

        return (T) PropertyDataFetcherHelper.getPropertyValue(propertyName, source, environment.getFieldType(), environment);
    }

    /**
     * PropertyDataFetcher caches the methods and fields that map from a class to a property for runtime performance reasons
     * as well as negative misses.
     * 
     * However during development you might be using an assistance tool like JRebel to allow you to tweak your code base and this
     * caching may interfere with this.  So you can call this method to clear the cache.  A JRebel plugin could
     * be developed to do just that.
     */
    @SuppressWarnings("unused")
    public static void clearReflectionCache() {
        PropertyDataFetcherHelper.clearReflectionCache();
    }

    /**
     * This can be used to control whether PropertyDataFetcher will use {@link java.lang.reflect.Method#setAccessible(boolean)} to gain access to property
     * values.  By default it PropertyDataFetcher WILL use setAccessible.
     *
     * @param flag whether to use setAccessible
     * @return the previous value of the flag
     */
    public static boolean setUseSetAccessible(boolean flag) {
        return PropertyDataFetcherHelper.setUseSetAccessible(flag);
    }

    /**
     * This can be used to control whether PropertyDataFetcher will cache negative lookups for a property for performance reasons.  By default it PropertyDataFetcher WILL cache misses.
     *
     * @param flag whether to cache misses
     * @return the previous value of the flag
     */
    public static boolean setUseNegativeCache(boolean flag) {
        return PropertyDataFetcherHelper.setUseNegativeCache(flag);
    }
}