NIFI-3206: Add ifElse function to Expression Language

This closes #1333

Signed-off-by: jpercivall <JPercivall@apache.org>

nifi-commons/nifi-expression-language/src/main/antlr3/org/apache/nifi/attribute/expression/language/antlr/AttributeExpressionLexer.g

@@ -182,6 +182,7 @@ SUBSTRING	: 'substring';
 REPLACE	: 'replace';
 REPLACE_FIRST	: 'replaceFirst';
 REPLACE_ALL : 'replaceAll';
+IF_ELSE : 'ifElse';
 
 // 4 arg functions
 GET_DELIMITED_FIELD	: 'getDelimitedField';

nifi-commons/nifi-expression-language/src/main/antlr3/org/apache/nifi/attribute/expression/language/antlr/AttributeExpressionParser.g

@@ -77,7 +77,7 @@ zeroArgString : (TO_UPPER | TO_LOWER | TRIM | TO_STRING | URL_ENCODE | URL_DECOD
 oneArgString : ((SUBSTRING_BEFORE | SUBSTRING_BEFORE_LAST | SUBSTRING_AFTER | SUBSTRING_AFTER_LAST | REPLACE_NULL | REPLACE_EMPTY |
 				PREPEND | APPEND | FORMAT | STARTS_WITH | ENDS_WITH | CONTAINS | JOIN | JSON_PATH | FROM_RADIX) LPAREN! anyArg RPAREN!) |
 			   (TO_RADIX LPAREN! anyArg (COMMA! anyArg)? RPAREN!);
-twoArgString : ((REPLACE | REPLACE_FIRST | REPLACE_ALL) LPAREN! anyArg COMMA! anyArg RPAREN!) |
+twoArgString : ((REPLACE | REPLACE_FIRST | REPLACE_ALL | IF_ELSE) LPAREN! anyArg COMMA! anyArg RPAREN!) |
 			   (SUBSTRING LPAREN! anyArg (COMMA! anyArg)? RPAREN!);
 fiveArgString : GET_DELIMITED_FIELD LPAREN! anyArg (COMMA! anyArg (COMMA! anyArg (COMMA! anyArg (COMMA! anyArg)?)?)?)? RPAREN!;
 

nifi-commons/nifi-expression-language/src/main/java/org/apache/nifi/attribute/expression/language/Query.java

@@ -54,6 +54,7 @@ import org.apache.nifi.attribute.expression.language.evaluation.functions.Greate
 import org.apache.nifi.attribute.expression.language.evaluation.functions.GreaterThanOrEqualEvaluator;
 import org.apache.nifi.attribute.expression.language.evaluation.functions.HostnameEvaluator;
 import org.apache.nifi.attribute.expression.language.evaluation.functions.IPEvaluator;
+import org.apache.nifi.attribute.expression.language.evaluation.functions.IfElseEvaluator;
 import org.apache.nifi.attribute.expression.language.evaluation.functions.InEvaluator;
 import org.apache.nifi.attribute.expression.language.evaluation.functions.IndexOfEvaluator;
 import org.apache.nifi.attribute.expression.language.evaluation.functions.IsEmptyEvaluator;
@@ -126,6 +127,7 @@ import org.antlr.runtime.CommonTokenStream;
 import org.antlr.runtime.tree.Tree;
 
 import static org.apache.nifi.attribute.expression.language.antlr.AttributeExpressionParser.FROM_RADIX;
+import static org.apache.nifi.attribute.expression.language.antlr.AttributeExpressionParser.IF_ELSE;
 import static org.apache.nifi.attribute.expression.language.antlr.AttributeExpressionParser.MATH;
 import static org.apache.nifi.attribute.expression.language.antlr.AttributeExpressionParser.ALL_ATTRIBUTES;
 import static org.apache.nifi.attribute.expression.language.antlr.AttributeExpressionParser.ALL_DELINEATED_VALUES;
@@ -1361,6 +1363,12 @@ public class Query {
                 return addToken(new JsonPathEvaluator(toStringEvaluator(subjectEvaluator),
                         toStringEvaluator(argEvaluators.get(0), "first argument to jsonPath")), "jsonPath");
             }
+            case IF_ELSE: {
+                verifyArgCount(argEvaluators, 2, "ifElse");
+                return addToken(new IfElseEvaluator(toBooleanEvaluator(subjectEvaluator),
+                        toStringEvaluator(argEvaluators.get(0), "argument to return if true"),
+                        toStringEvaluator(argEvaluators.get(1), "argument to return if false")), "ifElse");
+            }
             default:
                 throw new AttributeExpressionLanguageParsingException("Expected a Function-type expression but got " + tree.toString());
             }

nifi-commons/nifi-expression-language/src/main/java/org/apache/nifi/attribute/expression/language/evaluation/functions/IfElseEvaluator.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.nifi.attribute.expression.language.evaluation.functions;
+
+import org.apache.nifi.attribute.expression.language.evaluation.Evaluator;
+import org.apache.nifi.attribute.expression.language.evaluation.QueryResult;
+import org.apache.nifi.attribute.expression.language.evaluation.StringEvaluator;
+import org.apache.nifi.attribute.expression.language.evaluation.StringQueryResult;
+
+import java.util.Map;
+
+public class IfElseEvaluator extends StringEvaluator {
+
+    private final Evaluator<Boolean> subject;
+    private final Evaluator<String> trueEvaluator;
+    private final Evaluator<String> falseEvaluator;
+
+    public IfElseEvaluator(final Evaluator<Boolean> subject, final Evaluator<String> trueEvaluator, final Evaluator<String> falseEvaluator) {
+        this.subject = subject;
+        this.trueEvaluator = trueEvaluator;
+        this.falseEvaluator = falseEvaluator;
+    }
+
+    @Override
+    public QueryResult<String> evaluate(final Map<String, String> attributes) {
+        final QueryResult<Boolean> subjectValue = subject.evaluate(attributes);
+        if (subjectValue == null) {
+            return new StringQueryResult(null);
+        }
+        final String ifElseValue = (Boolean.TRUE.equals(subjectValue.getValue())) ? trueEvaluator.evaluate(attributes).getValue() : falseEvaluator.evaluate(attributes).getValue();
+        return new StringQueryResult(ifElseValue);
+    }
+
+    @Override
+    public Evaluator<?> getSubjectEvaluator() {
+        return subject;
+    }
+
+}

nifi-commons/nifi-expression-language/src/test/java/org/apache/nifi/attribute/expression/language/TestQuery.java

@@ -1684,6 +1684,23 @@ public class TestQuery {
           verifyEquals("${string:unescapeHtml4()}", attributes, "special ♣");
         }
 
+    @Test
+    public void testIfElse() {
+        final Map<String, String> attributes = new HashMap<>();
+        verifyEquals("${attr:isNull():ifElse('a', 'b')}", attributes, "a");
+        verifyEquals("${attr:ifElse('a', 'b')}", attributes, "b");
+        attributes.put("attr", "hello");
+        verifyEquals("${attr:isNull():ifElse('a', 'b')}", attributes, "b");
+        verifyEquals("${attr:ifElse('a', 'b')}", attributes, "b");
+        attributes.put("attr", "true");
+        verifyEquals("${attr:ifElse('a', 'b')}", attributes, "a");
+
+        verifyEquals("${attr2:isNull():ifElse('a', 'b')}", attributes, "a");
+        verifyEquals("${literal(true):ifElse('a', 'b')}", attributes, "a");
+        verifyEquals("${literal(true):ifElse(false, 'b')}", attributes, "false");
+
+    }
+
     private void verifyEquals(final String expression, final Map<String, String> attributes, final Object expectedResult) {
         verifyEquals(expression,attributes, null, expectedResult);
     }

nifi-docs/src/main/asciidoc/expression-language-guide.adoc

@@ -228,7 +228,7 @@ interpreted as literals. For example these two expressions are valid (without th
 One of the most powerful features of the Expression Language is the ability to compare an attribute value against
 some other value. This is used often, for example, to configure how a Processor should route data. The following
 functions are used for performing boolean logic, such as comparing two values.
-Each of these functions returns a value of type Boolean.
+Each of these functions are designed to work on values of type Boolean.
 
 
 [.function]
@@ -480,6 +480,37 @@ ${filename:toLower():equals( ${filename} ):or(
 
 
 
+[.function]
+=== ifElse
+
+*Description*: [.description]#Evaluates the first argument if the Subject evaluates to true, or the second argument
+if the Subject evaluates to false.#
+
+*Subject Type*: [.subject]#Boolean#
+
+*Arguments*:
+
+	- [.argName]#_EvaluateIfTrue_# : [.argDesc]#The value to return if the Subject is true#
+	- [.argName]#_EvaluateIfFalse_# : [.argDesc]#The value to return if the Subject is false#
+
+*Return Type*: [.returnType]#String#
+
+*Examples*: If the "filename" attribute has the value "a brand new filename.txt", the "nullFilename" attribute has
+the value null, and the "bool" attribute has the value "true", then the following expressions will provide
+the following results:
+
+
+
+.ifElse Examples
+|===================================================================
+| Expression | Value
+| `${bool:ifElse('a','b')}` | `a`
+| `${literal(true):ifElse('a','b')}` | `a`
+| `${nullFilename:isNull():ifElse('file does not exist', 'located file')}` | `file does not exist`
+| `${nullFilename:ifElse('found', 'not_found')}` | `not_found`
+| `${filename:ifElse('found', 'not_found')}` | `not_found`
+| `${filename:isNull():not():ifElse('found', 'not_found')}` | `found`
+|===================================================================
 
 
 
@@ -963,10 +994,6 @@ Expressions will provide the following results:
 	will return 0.
 
 
-
-
-
-
 [[encode]]
 == Encode/Decode Functions