Fix #5194: allow serializing Throwables even with custom visibility settings (#5220)

Author: cowtowncoder

pom.xml

@@ -391,7 +391,9 @@
         <jdk>17</jdk>
       </activation>
       <properties>
-        <argLine>--add-opens=java.base/java.util=ALL-UNNAMED</argLine>
+        <argLine>--add-opens=java.base/java.lang=ALL-UNNAMED
+	         --add-opens=java.base/java.util=ALL-UNNAMED
+	</argLine>
       </properties>
       <build>
         <plugins>
@@ -454,7 +456,9 @@
         <jdk>[21,)</jdk>
       </activation>
       <properties>
-        <argLine>--add-opens=java.base/java.util=ALL-UNNAMED</argLine>
+        <argLine>--add-opens=java.base/java.lang=ALL-UNNAMED
+	         --add-opens=java.base/java.util=ALL-UNNAMED
+	</argLine>
       </properties>
       <build>
         <plugins>

release-notes/VERSION-2.x

@@ -22,6 +22,9 @@ Project: jackson-databind
 #5192: Record types are broken on Android when using R8
  (reported by @HelloOO7)
  (fix by @pjfanning)
+#5194: Custom `Throwable` not serializable if using `JsonAutoDetect` settings
+ that only detect Fields
+ (reported by @riskop)
 #5197: Add more informative exception for back-references with `record` type
  (fix by Joo-Hyuk K)
 - Generate SBOMs [JSTEP-14]

src/main/java/com/fasterxml/jackson/databind/ser/BeanPropertyWriter.java

@@ -944,15 +944,27 @@ protected boolean _handleSelfReference(Object bean, JsonGenerator gen,
             throws IOException
     {
         if (!ser.usesObjectId()) {
+            boolean writeAsNull = false;
+
             if (prov.isEnabled(SerializationFeature.FAIL_ON_SELF_REFERENCES)) {
                 // 05-Feb-2013, tatu: Usually a problem, but NOT if we are handling
                 // object id; this may be the case for BeanSerializers at least.
                 // 13-Feb-2014, tatu: another possible ok case: custom serializer
                 // (something OTHER than {@link BeanSerializerBase}
                 if (ser instanceof BeanSerializerBase) {
-                    prov.reportBadDefinition(getType(), "Direct self-reference leading to cycle");
+                    // 09-Jul-2025, tatu: [databind#5194] Let's suppress specific case
+                    //   of "cause" for Throwables
+                    if (_isThrowableFieldCause(prov, bean)) {
+                        writeAsNull = true;
+                    } else {
+                        prov.reportBadDefinition(getType(), "Direct self-reference leading to cycle");
+                    }
                 }
-            } else if (prov.isEnabled(SerializationFeature.WRITE_SELF_REFERENCES_AS_NULL)) {
+            } else {
+                writeAsNull = prov.isEnabled(SerializationFeature.WRITE_SELF_REFERENCES_AS_NULL);
+            }
+
+            if (writeAsNull) {
                 if (_nullSerializer != null) {
                     // 23-Oct-2019, tatu: Tricky part -- caller does not specify if it's
                     //   "as property" (in JSON Object) or "as element" (JSON array, via
@@ -970,6 +982,17 @@ protected boolean _handleSelfReference(Object bean, JsonGenerator gen,
         return false;
     }
 
+    // Helper method to recognize `Throwable.cause` Field, which has "this" as initialized
+    // value to mean "not set" (`Throwable.getCause()` translates this to `null`).
+    //
+    // @since 2.20
+    private boolean _isThrowableFieldCause(SerializerProvider prov, Object bean) {
+        return bean instanceof Throwable
+                && _member instanceof AnnotatedField
+                && "cause".equals(_name.getValue())
+                ;
+    }
+    
     @Override
     public String toString() {
         StringBuilder sb = new StringBuilder(40);

src/test/java/com/fasterxml/jackson/databind/ser/jdk/CustomExceptionSer5194Test.java

@@ -0,0 +1,47 @@
+package com.fasterxml.jackson.databind.ser.jdk;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.annotation.JsonAutoDetect;
+import com.fasterxml.jackson.annotation.PropertyAccessor;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.json.JsonMapper;
+import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+public class CustomExceptionSer5194Test
+    extends DatabindTestUtil
+{
+    static class MyIllegalArgumentException extends RuntimeException {
+        private static final long serialVersionUID = 1L;
+
+        public MyIllegalArgumentException() {
+            super();
+        }
+
+        public MyIllegalArgumentException(String s) {
+            super(s);
+        }
+
+        public MyIllegalArgumentException(String message, Throwable cause) {
+            super(message, cause);
+        }
+
+        public MyIllegalArgumentException(Throwable cause) {
+            super(cause);
+        }
+    }
+
+    // [databind#5194]: failed to serialize custom exception
+    @Test
+    public void test5194() throws Exception {
+        ObjectMapper mapper = JsonMapper.builder()
+                .visibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.NONE)
+                .visibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY)
+                .build();
+
+        String json = mapper.writeValueAsString(new MyIllegalArgumentException());
+        assertNotNull(json);
+    }
+}