fix: body access for SpEL e.g. ${body} (membrane#1698)

* fix: body access for SpEL e.g. ${body}

* refactor:  minor

* return error in case body could not be decoded

---------

Co-authored-by: Tobias Polley <mail@tobias-polley.de>

Author: predic8

core/src/main/java/com/predic8/membrane/core/lang/spel/SpELExchangeEvaluationContext.java

@@ -39,6 +39,7 @@ public class SpELExchangeEvaluationContext extends StandardEvaluationContext {
 
     private final Exchange exchange;
     private final Message message;
+    private final SpELBody body; // Is used by SpEL in scripts
 
     // Avoid the common plural error
     private final SpELLablePropertyAware headers;
@@ -55,7 +56,7 @@ public class SpELExchangeEvaluationContext extends StandardEvaluationContext {
 
     private int statusCode;
 
-    private String scopes;
+    private String scopes; // Is used by SpEL in scripts
 
     private SpELMessageWrapper request;
     private SpELMessageWrapper response;
@@ -67,6 +68,7 @@ public SpELExchangeEvaluationContext(Exchange exchange, Flow flow) {
         this.exchange = exchange;
         this.flow = flow;
         this.message = exchange.getMessage(flow);
+        this.body = new SpELBody(message);
 
         pathParam = new SpELPathParameters(exchange);
         properties = new SpELProperties(exchange.getProperties());
@@ -88,6 +90,7 @@ private void addTypeConverters() {
         GenericConversionService cs = new DefaultConversionService();
         cs.addConverter(new SpELHeaderToStringTypeConverter());
         cs.addConverter(new SpELMapToStringTypeConverter());
+        cs.addConverter(new SpELBodyToStringTypeConverter());
         setTypeConverter(new StandardTypeConverter(cs));
     }
 
@@ -154,6 +157,8 @@ public Message getMessage() {
         return message;
     }
 
+    public SpELBody getBody() { return body; }
+
     public String getPath() {
         return path;
     }

core/src/main/java/com/predic8/membrane/core/lang/spel/spelable/SpELBody.java

@@ -0,0 +1,32 @@
+/* Copyright 2025 predic8 GmbH, www.predic8.com
+
+   Licensed 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 com.predic8.membrane.core.lang.spel.spelable;
+
+import com.predic8.membrane.core.http.*;
+
+public class SpELBody {
+
+    /**
+     * Store message instead of body to be able to extract even zipped bodies
+     */
+    Message message;
+
+    public SpELBody(Message msg) {
+        message = msg;
+    }
+
+    public Message getMessage() {
+        return message;
+    }
+}

core/src/main/java/com/predic8/membrane/core/lang/spel/typeconverters/SpELBodyToStringTypeConverter.java

@@ -0,0 +1,35 @@
+/* Copyright 2025 predic8 GmbH, www.predic8.com
+
+   Licensed 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 com.predic8.membrane.core.lang.spel.typeconverters;
+
+import com.predic8.membrane.core.lang.spel.spelable.*;
+import com.predic8.membrane.core.util.*;
+import org.slf4j.*;
+import org.springframework.core.convert.converter.*;
+
+public class SpELBodyToStringTypeConverter implements Converter<SpELBody, String> {
+
+    private static final Logger log = LoggerFactory.getLogger(SpELBodyToStringTypeConverter.class.getName());
+
+    @Override
+    public String convert(SpELBody body) {
+        try {
+            return new String(MessageUtil.getContent(body.getMessage()));
+        } catch (Exception e) {
+            log.warn("Cannot log body content", e);
+            throw new RuntimeException(e);
+        }
+
+    }
+}

core/src/test/java/com/predic8/membrane/core/lang/spel/SpELExchangeEvaluationContextTest.java

@@ -159,4 +159,9 @@ void json() {
     void jsonUnknown() {
         assertEquals("", keyExpression("json['unknown']"));
     }
+
+    @Test
+    void body() {
+        assertTrue(keyExpression("body").contains("Snake oil"));
+    }
 }