CAMEL-22884: Validate method attribute placement in when clause

core/camel-core-model/src/main/java/org/apache/camel/model/BasicOutputExpressionNode.java

@@ -75,4 +75,32 @@ public void setOutputs(List<ProcessorDefinition<?>> outputs) {
     public void addOutput(ProcessorDefinition<?> output) {
         this.outputs.add(output);
     }
+
+    @Override
+    public void setExpression(ExpressionDefinition expression) {
+        // Detect when an expression element (e.g. <method>) appears after processing steps
+        // inside a when/filter clause in XML/YAML DSL. This is almost certainly a user mistake
+        // where they intended to use <bean> (processor) instead of <method> (expression/predicate).
+        // We skip this check when the existing expression wraps an ExpressionClause (Java DSL),
+        // because preCreateProcessor() legitimately re-sets the expression after resolving it.
+        if (expression != null && getExpression() != null && !outputs.isEmpty()) {
+            ExpressionDefinition existing = getExpression();
+            boolean isExpressionClause
+                    = existing.getExpressionValue() instanceof org.apache.camel.builder.ExpressionClause
+                            || existing.getPredicate() instanceof org.apache.camel.builder.ExpressionClause;
+            if (!isExpressionClause) {
+                String lang = expression.getLanguage() != null
+                        ? expression.getLanguage() : expression.getClass().getSimpleName();
+                throw new IllegalArgumentException(
+                        "The " + getShortName() + " already has a predicate (" + existing
+                                                   + ") and " + outputs.size() + " output(s). "
+                                                   + "The expression '" + lang
+                                                   + "' is being parsed as an expression/predicate but appears after processing steps. "
+                                                   + "If you intended to call a bean method as a processing step, use <bean> instead of <method>. "
+                                                   + "An expression element must be the first child of <" + getShortName()
+                                                   + ">.");
+            }
+        }
+        super.setExpression(expression);
+    }
 }

core/camel-xml-io/src/test/java/org/apache/camel/xml/in/ModelParserTest.java

@@ -449,6 +449,52 @@ public void testParseError() throws Exception {
         assertTrue(e.getMessage().startsWith("Unexpected attribute '{}ref'"));
     }
 
+    @Test
+    public void testMethodAfterStepsInWhenClauseShouldFail() throws Exception {
+        String routesXml = "<routes xmlns=\"http://camel.apache.org/schema/xml-io\">"
+                           + "  <route>"
+                           + "    <from uri=\"direct:start\"/>"
+                           + "    <choice>"
+                           + "      <when>"
+                           + "        <simple>${header.foo} == 'bar'</simple>"
+                           + "        <log message=\"Condition met\"/>"
+                           + "        <method ref=\"someBean\" method=\"processFooBar\"/>"
+                           + "      </when>"
+                           + "      <otherwise>"
+                           + "        <to uri=\"mock:other\"/>"
+                           + "      </otherwise>"
+                           + "    </choice>"
+                           + "  </route>"
+                           + "</routes>";
+        Exception e = assertThrows(Exception.class, () -> {
+            new ModelParser(new StringReader(routesXml)).parseRoutesDefinition();
+        });
+        assertTrue(e.getMessage().contains("already has a predicate")
+                || e.getCause().getMessage().contains("already has a predicate"),
+                "Error message should indicate that the when clause already has a predicate: " + e.getMessage());
+    }
+
+    @Test
+    public void testMethodAsPredicateInWhenClauseShouldWork() throws Exception {
+        String routesXml = "<routes xmlns=\"http://camel.apache.org/schema/xml-io\">"
+                           + "  <route>"
+                           + "    <from uri=\"direct:start\"/>"
+                           + "    <choice>"
+                           + "      <when>"
+                           + "        <method ref=\"someBean\" method=\"isFoo\"/>"
+                           + "        <to uri=\"mock:foo\"/>"
+                           + "      </when>"
+                           + "      <otherwise>"
+                           + "        <to uri=\"mock:other\"/>"
+                           + "      </otherwise>"
+                           + "    </choice>"
+                           + "  </route>"
+                           + "</routes>";
+        RoutesDefinition routes = new ModelParser(new StringReader(routesXml)).parseRoutesDefinition().orElse(null);
+        assertNotNull(routes);
+        assertEquals(1, routes.getRoutes().size());
+    }
+
     private Path getResourceFolder() {
         final URL resource = getClass().getClassLoader().getResource("barInterceptorRoute.xml");
         assert resource != null : "Cannot find barInterceptorRoute.xml";