Added Java example code

Author: bbakerman

readme.md

@@ -106,23 +106,43 @@ will use that as a resource bundle lookup key.  This too is inspired by the java
 
 Like javax.validation, this library ships with some default error message templates but you can override them. 
 
+# Schema Directive Wiring
+
+If you are using graphql SDL to build your graphql schema then you can use a `ValidationSchemaWiring` class to automatically
+change your field data fetchers so that validation rules are called before the field data is fetched.
+
+This allows you to automatically enhance your schema with validation by directives alone.
+
+The following shows how to setup the SDL generation so that the build schema will have validation in place.
+
+
+```java
+        //
+        // This contains by default the standard library provided @Directive constraints
+        //
+        PossibleValidationRules possibleRules = PossibleValidationRules.newPossibleRules()
+                .onValidationErrorStrategy(OnValidationErrorStrategy.RETURN_NULL)
+                .build();
+        //
+        // This will rewrite your data fetchers when rules apply to them so that validation
+        ValidationSchemaWiring schemaWiring = new ValidationSchemaWiring(possibleRules);
+        //
+        // we add this schema wiring to the graphql runtime
+        RuntimeWiring runtimeWiring = RuntimeWiring.newRuntimeWiring().directiveWiring(schemaWiring).build();
+        //
+        // then pretty much standard graphql-java code to build a graphql schema
+        Reader sdl = buildSDL();
+        TypeDefinitionRegistry typeDefinitionRegistry = new SchemaParser().parse(sdl);
+        GraphQLSchema graphQLSchema = new SchemaGenerator().makeExecutableSchema(typeDefinitionRegistry, runtimeWiring);
+```
 
-# Complex input types
-
-You can put @Directive constraints on complex input types as well as simple field arguments
-
-```graphql
-    input ProductItem {
-        code : String @Size(max : 5)
-        price : String @Size(max : 3)
-    }
+Under the covers `ValidationSchemaWiring` asks each possible rule if it applies to a field as they are encountered (at schema build time).
 
-    type Mutation {
-            updateProduct( product : ID,  items : [ProductItem]) : Product
-    }   
-```
+If they do apply then it rewrites the DataFetcher so that it first calls the validation code and produces errors if the field input is not 
+considered valid.
 
-In the example above each `ProductItem` in the list of items is examined for valid values
+The default strategy `OnValidationErrorStrategy.RETURN_NULL` will return null for the field input if it is not considered valid.  You can 
+write your own strategy if you want.
 
 ## The supplied @Directive constraints
 

src/test/java/examples/SchemaWiringExample.java

@@ -0,0 +1,73 @@
+package examples;
+
+import graphql.ExecutionInput;
+import graphql.ExecutionResult;
+import graphql.GraphQL;
+import graphql.schema.GraphQLSchema;
+import graphql.schema.idl.RuntimeWiring;
+import graphql.schema.idl.SchemaGenerator;
+import graphql.schema.idl.SchemaParser;
+import graphql.schema.idl.TypeDefinitionRegistry;
+import graphql.validation.rules.OnValidationErrorStrategy;
+import graphql.validation.rules.PossibleValidationRules;
+import graphql.validation.schemawiring.ValidationSchemaWiring;
+
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.util.HashMap;
+import java.util.Map;
+
+@SuppressWarnings("UnnecessaryLocalVariable")
+public class SchemaWiringExample {
+
+    public static void main(String[] args) {
+
+        try {
+            GraphQLSchema graphQLSchema = buildSchemaAndWiring();
+            GraphQL graphQL = GraphQL.newGraphQL(graphQLSchema).build();
+
+            Map<String, Object> variables = new HashMap<>();
+            variables.put("x", "y");
+
+            ExecutionInput ei = ExecutionInput.newExecutionInput()
+                    .query("{field}")
+                    .variables(variables)
+                    .build();
+
+            ExecutionResult result = graphQL.execute(ei);
+
+            System.out.println(result);
+        } catch (RuntimeException rte) {
+            rte.printStackTrace(System.out);
+        }
+
+    }
+
+    private static GraphQLSchema buildSchemaAndWiring() {
+        //
+        // This contains by default the standard library provided @Directive constraints
+        //
+        PossibleValidationRules possibleRules = PossibleValidationRules.newPossibleRules()
+                .onValidationErrorStrategy(OnValidationErrorStrategy.RETURN_NULL)
+                .build();
+        //
+        // This will rewrite your data fetchers when rules apply to them so that validation
+        ValidationSchemaWiring schemaWiring = new ValidationSchemaWiring(possibleRules);
+        //
+        // we add this schema wiring to the graphql runtime
+        RuntimeWiring runtimeWiring = RuntimeWiring.newRuntimeWiring().directiveWiring(schemaWiring).build();
+        //
+        // then pretty much standard graphql-java code to build a graphql schema
+        Reader sdl = buildSDL();
+        TypeDefinitionRegistry typeDefinitionRegistry = new SchemaParser().parse(sdl);
+        GraphQLSchema graphQLSchema = new SchemaGenerator().makeExecutableSchema(typeDefinitionRegistry, runtimeWiring);
+        
+        return graphQLSchema;
+    }
+
+    private static Reader buildSDL() {
+        InputStream is = SchemaWiringExample.class.getResourceAsStream("/examples/example.graphqls");
+        return new InputStreamReader(is);
+    }
+}

src/test/resources/examples/example.graphqls

@@ -0,0 +1,6 @@
+
+
+
+type Query {
+    field : String
+}