Document safe navigation semantics within compound expressions in SpEL

Closes gh-21827

Author: sbrannen

framework-docs/modules/ROOT/pages/core/expressions/language-ref/operator-safe-navigation.adoc

@@ -5,7 +5,17 @@ The safe navigation operator (`?`) is used to avoid a `NullPointerException` and
 from the https://www.groovy-lang.org/operators.html#_safe_navigation_operator[Groovy]
 language. Typically, when you have a reference to an object, you might need to verify
 that it is not `null` before accessing methods or properties of the object. To avoid
-this, the safe navigation operator returns `null` instead of throwing an exception.
+this, the safe navigation operator returns `null` for the particular null-safe operation
+instead of throwing an exception.
+
+[WARNING]
+====
+When the safe navigation operator evaluates to `null` for a particular null-safe
+operation within a compound expression, the remainder of the compound expression will
+still be evaluated.
+
+See <<expressions-operator-safe-navigation-compound-expressions>> for details.
+====
 
 [[expressions-operator-safe-navigation-property-access]]
 == Safe Property and Method Access
@@ -283,3 +293,71 @@ Kotlin::
 ======
 
 
+[[expressions-operator-safe-navigation-compound-expressions]]
+== Null-safe Operations in Compound Expressions
+
+As mentioned at the beginning of this section, when the safe navigation operator
+evaluates to `null` for a particular null-safe operation within a compound expression,
+the remainder of the compound expression will still be evaluated. This means that the
+safe navigation operator must be applied throughout a compound expression in order to
+avoid any unwanted `NullPointerException`.
+
+Given the expression `#person?.address.city`, if `#person` is `null` the safe navigation
+operator (`?.`) ensures that no exception will be thrown when attempting to access the
+`address` property of `#person`. However, since `#person?.address` evaluates to `null`, a
+`NullPointerException` will be thrown when attempting to access the `city` property of
+`null`. To address that, you can apply null-safe navigation throughout the compound
+expression as in `#person?.address?.city`. That expression will safely evaluate to `null`
+if either `#person` or `#person?.address` evaluates to `null`.
+
+The following example demonstrates how to use the "null-safe select first" operator
+(`?.^`) on a collection combined with null-safe property access (`?.`) within a compound
+expression. If `members` is `null`, the result of the "null-safe select first" operator
+(`members?.^[nationality == 'Serbian']`) evaluates to `null`, and the additional use of
+the safe navigation operator (`?.name`) ensures that the entire compound expression
+evaluates to `null` instead of throwing an exception.
+
+[tabs]
+======
+Java::
++
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+----
+	ExpressionParser parser = new SpelExpressionParser();
+	IEEE society = new IEEE();
+	StandardEvaluationContext context = new StandardEvaluationContext(society);
+	String expression = "members?.^[nationality == 'Serbian']?.name"; // <1>
+
+	// evaluates to "Nikola Tesla"
+	String name = parser.parseExpression(expression)
+			.getValue(context, String.class);
+
+	society.members = null;
+
+	// evaluates to null - does not throw a NullPointerException
+	name = parser.parseExpression(expression)
+			.getValue(context, String.class);
+----
+<1> Use "null-safe select first" and null-safe property access operators within compound expression.
+
+Kotlin::
++
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+----
+	val parser = SpelExpressionParser()
+	val society = IEEE()
+	val context = StandardEvaluationContext(society)
+	val expression = "members?.^[nationality == 'Serbian']?.name" // <1>
+
+	// evaluates to "Nikola Tesla"
+	String name = parser.parseExpression(expression)
+			.getValue(context, String::class.java)
+
+	society.members = null
+
+	// evaluates to null - does not throw a NullPointerException
+	name = parser.parseExpression(expression)
+			.getValue(context, String::class.java)
+----
+<1> Use "null-safe select first" and null-safe property access operators within compound expression.
+======

spring-expression/src/test/java/org/springframework/expression/spel/SpelDocumentationTests.java

@@ -689,6 +689,25 @@ void nullSafeSelectFirst() {
 			assertThat(inventor).isNull();
 		}
 
+		@Test
+		void nullSafeSelectFirstAndPropertyAccess() {
+			IEEE society = new IEEE();
+			StandardEvaluationContext context = new StandardEvaluationContext(society);
+			String expression = "members?.^[nationality == 'Serbian']?.name"; // <1>
+
+			// evaluates to "Nikola Tesla"
+			String name = parser.parseExpression(expression)
+					.getValue(context, String.class);
+			assertThat(name).isEqualTo("Nikola Tesla");
+
+			society.members = null;
+
+			// evaluates to null - does not throw a NullPointerException
+			name = parser.parseExpression(expression)
+					.getValue(context, String.class);
+			assertThat(name).isNull();
+		}
+
 		@Test
 		@SuppressWarnings("unchecked")
 		void nullSafeSelectLast() {