Jackson is "the Java JSON library" or "the best JSON parser for Java". Or simply as "JSON for Java".

Jackson Databind Source Code files are provided in the source packge (jackson-databind-2.14.0-sources.jar). You can download it at Jackson Maven Website.

You can also browse Jackson Databind Source Code below:

⏎ com/fasterxml/jackson/databind/introspect/MethodGenericTypeResolver.java

package com.fasterxml.jackson.databind.introspect;

import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.type.TypeBindings;
import com.fasterxml.jackson.databind.type.TypeFactory;

import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.lang.reflect.WildcardType;
import java.util.ArrayList;
import java.util.Objects;

/**
 * Internal utility functionality to handle type resolution for method type variables
 * based on the requested result type: this is needed to work around the problem of
 * static factory methods not being able to use class type variable bindings
 * (see [databind#2895] for more).
 *
 * @since 2.12
 */
final class MethodGenericTypeResolver
{
    /*
     * Attempt to narrow types on a generic factory method based on the expected result (requestedType).
     * If narrowing was possible, a new TypeResolutionContext is returned with the discovered TypeBindings,
     * otherwise the emptyTypeResCtxt argument is returned.
     *
     * For example:
     * Given type Wrapper<T> with
     * @JsonCreator static <T> Wrapper<T> fromJson(T value)
     * When a Wrapper<Duck> is requested the factory must return a Wrapper<Duck> and we can bind T to Duck
     * as though the method was written with defined types:
     * @JsonCreator static Wrapper<Duck> fromJson(Duck value)
     */
    public static TypeResolutionContext narrowMethodTypeParameters(
            Method candidate,
            JavaType requestedType,
            TypeFactory typeFactory,
            TypeResolutionContext emptyTypeResCtxt) {
        TypeBindings newTypeBindings = bindMethodTypeParameters(candidate, requestedType, emptyTypeResCtxt);
        return newTypeBindings == null
                ? emptyTypeResCtxt
                : new TypeResolutionContext.Basic(typeFactory, newTypeBindings);
    }

    /**
     * Returns {@link TypeBindings} with additional type information
     * based on {@code requestedType} if possible, otherwise {@code null}.
     */
    static TypeBindings bindMethodTypeParameters(
            Method candidate,
            JavaType requestedType,
            TypeResolutionContext emptyTypeResCtxt) {
        TypeVariable<Method>[] methodTypeParameters = candidate.getTypeParameters();
        if (methodTypeParameters.length == 0
                // If the primary type has no type parameters, there's nothing to do
                || requestedType.getBindings().isEmpty()) {
            // Method has no type parameters: no need to modify the resolution context.
            return null;
        }
        Type genericReturnType = candidate.getGenericReturnType();
        if (!(genericReturnType instanceof ParameterizedType)) {
            // Return value is not parameterized, it cannot be used to associate the requestedType expectations
            // onto parameters.
            return null;
        }

        ParameterizedType parameterizedGenericReturnType = (ParameterizedType) genericReturnType;
        // Primary type and result type must be the same class, otherwise we would need to
        // trace generic parameters to a common superclass or interface.
        if (!Objects.equals(requestedType.getRawClass(), parameterizedGenericReturnType.getRawType())) {
            return null;
        }

        // Construct TypeBindings based on the requested type, and type variables that occur in the generic return type.
        // For example given requestedType: Foo<String, Int>
        // and method static <T, U> Foo<T, U> func(Bar<T, U> in)
        // Produces TypeBindings{T=String, U=Int}.
        Type[] methodReturnTypeArguments = parameterizedGenericReturnType.getActualTypeArguments();
        ArrayList<String> names = new ArrayList<>(methodTypeParameters.length);
        ArrayList<JavaType> types = new ArrayList<>(methodTypeParameters.length);
        for (int i = 0; i < methodReturnTypeArguments.length; i++) {
            Type methodReturnTypeArgument = methodReturnTypeArguments[i];
            // Note: This strictly supports only TypeVariables of the forms "T" and "? extends T",
            // not complex wildcards with nested type variables
            TypeVariable<?> typeVar = maybeGetTypeVariable(methodReturnTypeArgument);
            if (typeVar != null) {
                String typeParameterName = typeVar.getName();
                if (typeParameterName == null) {
                    return null;
                }

                JavaType bindTarget = requestedType.getBindings().getBoundType(i);
                if (bindTarget == null) {
                    return null;
                }
                // If the type parameter name is not present in the method type parameters we
                // fall back to default type handling.
                TypeVariable<?> methodTypeVariable = findByName(methodTypeParameters, typeParameterName);
                if (methodTypeVariable == null) {
                    return null;
                }
                if (pessimisticallyValidateBounds(emptyTypeResCtxt, bindTarget, methodTypeVariable.getBounds())) {
                    // Avoid duplicate entries for the same type variable, e.g. '<T> Map<T, T> foo(Class<T> in)'
                    int existingIndex = names.indexOf(typeParameterName);
                    if (existingIndex != -1) {
                        JavaType existingBindTarget = types.get(existingIndex);
                        if (bindTarget.equals(existingBindTarget)) {
                            continue;
                        }
                        boolean existingIsSubtype = existingBindTarget.isTypeOrSubTypeOf(bindTarget.getRawClass());
                        boolean newIsSubtype = bindTarget.isTypeOrSubTypeOf(existingBindTarget.getRawClass());
                        if (!existingIsSubtype && !newIsSubtype) {
                            // No way to satisfy the requested type.
                            return null;
                        }
                        if ((existingIsSubtype ^ newIsSubtype) && newIsSubtype) {
                            // If the new type is more specific than the existing type, the new type replaces the old.
                            types.set(existingIndex, bindTarget);
                        }
                    } else {
                        names.add(typeParameterName);
                        types.add(bindTarget);
                    }
                }
            }
        }
        // Fall back to default handling if no specific types from the requestedType are used
        if (names.isEmpty()) {
            return null;
        }
        return TypeBindings.create(names, types);
    }

    /* Returns the TypeVariable if it can be extracted, otherwise null. */
    private static TypeVariable<?> maybeGetTypeVariable(Type type) {
        if (type instanceof TypeVariable) {
            return (TypeVariable<?>) type;
        }
        // Extract simple type variables from wildcards matching '? extends T'
        if (type instanceof WildcardType) {
            WildcardType wildcardType = (WildcardType) type;
            // Exclude any form of '? super T'
            if (wildcardType.getLowerBounds().length != 0) {
                return null;
            }
            Type[] upperBounds = wildcardType.getUpperBounds();
            if (upperBounds.length == 1) {
                return maybeGetTypeVariable(upperBounds[0]);
            }
        }
        return null;
    }

    /* Returns the TypeVariable if it can be extracted, otherwise null. */
    private static ParameterizedType maybeGetParameterizedType(Type type) {
        if (type instanceof ParameterizedType) {
            return (ParameterizedType) type;
        }
        // Extract simple type variables from wildcards matching '? extends T'
        if (type instanceof WildcardType) {
            WildcardType wildcardType = (WildcardType) type;
            // Exclude any form of '? super T'
            if (wildcardType.getLowerBounds().length != 0) {
                return null;
            }
            Type[] upperBounds = wildcardType.getUpperBounds();
            if (upperBounds.length == 1) {
                return maybeGetParameterizedType(upperBounds[0]);
            }
        }
        return null;
    }

    private static boolean pessimisticallyValidateBounds(
            TypeResolutionContext context, JavaType boundType, Type[] upperBound) {
        for (Type type : upperBound) {
            if (!pessimisticallyValidateBound(context, boundType, type)) {
                return false;
            }
        }
        return true;
    }

    private static boolean pessimisticallyValidateBound(
            TypeResolutionContext context, JavaType boundType, Type type) {
        if (!boundType.isTypeOrSubTypeOf(context.resolveType(type).getRawClass())) {
            return false;
        }
        ParameterizedType parameterized = maybeGetParameterizedType(type);
        if (parameterized != null
                // 09-Nov-2020, ckozak: Validate equivalent parameters if possible, however when types do not
                // exactly match, there's not much validation we can reasonably do.
                && Objects.equals(boundType.getRawClass(), parameterized.getRawType())) {
            Type[] typeArguments = parameterized.getActualTypeArguments();
            TypeBindings bindings = boundType.getBindings();
            if (bindings.size() != typeArguments.length) {
                return false;
            }
            for (int i = 0; i < bindings.size(); i++) {
                JavaType boundTypeBound = bindings.getBoundType(i);
                Type typeArg = typeArguments[i];
                if (!pessimisticallyValidateBound(context, boundTypeBound, typeArg)) {
                    return false;
                }
            }
        }
        return true;
    }

    private static TypeVariable<?> findByName(TypeVariable<?>[] typeVariables, String name) {
        if (typeVariables == null || name == null) {
            return null;
        }
        for (TypeVariable<?> typeVariable : typeVariables) {
            if (name.equals(typeVariable.getName())) {
                return typeVariable;
            }
        }
        return null;
    }
}

⏎ com/fasterxml/jackson/databind/introspect/MethodGenericTypeResolver.java

Or download all of them as a single archive file:

File name: jackson-databind-2.14.0-sources.jar
File size: 1187952 bytes
Release date: 2022-11-05
Download 

⇒ Jackson Annotations Source Code

⇐ Download and Install Jackson Binary Package

⇑ Downloading and Reviewing jackson-*.jar

⇑⇑ Jackson - Java JSON library