Fix #456: Do not call toString() of an object whose setter fails

The message of a failing setter concatenated the object under
deserialization, which calls its toString() on a partially deserialized
object. An exception from toString() then replaced the setter's. Name
the class instead, as the getter path does since #570.

Signed-off-by: Sharang Gupta <sharang@sharanggupta.dev>

src/main/java/org/eclipse/yasson/internal/deserializer/ValueSetterDeserializer.java

diff --git a/src/main/java/org/eclipse/yasson/internal/deserializer/ValueSetterDeserializer.java b/src/main/java/org/eclipse/yasson/internal/deserializer/ValueSetterDeserializer.java
index 857e55a..80f50d1 100644
--- a/src/main/java/org/eclipse/yasson/internal/deserializer/ValueSetterDeserializer.java
+++ b/src/main/java/org/eclipse/yasson/internal/deserializer/ValueSetterDeserializer.java
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021, 2022 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License v. 2.0 which is available at
@@ -32,12 +33,14 @@
 
     @Override
     public Object deserialize(Object value, DeserializationContextImpl context) {
-        Object object = context.getInstance();
+        Object instance = context.getInstance();
         try {
-            valueSetter.invoke(object, value);
+            valueSetter.invoke(instance, value);
             return value;
         } catch (Throwable e) {
-            throw new JsonbException("Error setting value on: " + object, e);
+            // Do not call toString() of the instance, which is only partially deserialized
+            String typeName = instance == null ? "null" : instance.getClass().getName();
+            throw new JsonbException("Error setting value on: " + typeName, e);
         }
     }
 

src/test/java/org/eclipse/yasson/defaultmapping/specific/DeserializationExceptionTest.java

diff --git a/src/test/java/org/eclipse/yasson/defaultmapping/specific/DeserializationExceptionTest.java b/src/test/java/org/eclipse/yasson/defaultmapping/specific/DeserializationExceptionTest.java
new file mode 100644
index 0000000..155f611
--- /dev/null
+++ b/src/test/java/org/eclipse/yasson/defaultmapping/specific/DeserializationExceptionTest.java
@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0,
+ * or the Eclipse Distribution License v. 1.0 which is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+ */
+
+package org.eclipse.yasson.defaultmapping.specific;
+
+import static org.eclipse.yasson.Jsonbs.defaultJsonb;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import jakarta.json.bind.JsonbException;
+
+/**
+ * Tests covering error-handling behaviour during deserialization: verifies that
+ * exceptions thrown by setter methods are reported as the cause of a {@link JsonbException},
+ * without calling {@code toString()} on the object under deserialization.
+ *
+ * @see <a href="https://github.com/eclipse-ee4j/yasson/issues/456">Issue #456</a>
+ */
+public class DeserializationExceptionTest {
+
+    private static final String JSON = "{\"iban\":\"XX\"}";
+
+    /**
+     * Tests that a {@link RuntimeException} thrown by a setter during deserialization is the
+     * root cause of the {@link JsonbException}.
+     *
+     * <p>The object is only partially deserialized at that point. Calling its {@code toString()}
+     * to build the error message used to replace the setter's exception with any exception
+     * thrown by {@code toString()}.
+     */
+    @Test
+    public void setterExceptionIsRootCause() {
+        List<Throwable> causes = causesOf(assertThrows(JsonbException.class,
+                                                       () -> defaultJsonb.fromJson(JSON, BrokenSetterPojo.class)));
+
+        Throwable rootCause = causes.get(causes.size() - 1);
+        assertEquals(IllegalArgumentException.class, rootCause.getClass());
+        assertEquals("simulated setter failure", rootCause.getMessage());
+    }
+
+    /**
+     * Tests that the error message of a failing setter names the class of the object.
+     */
+    @Test
+    public void setterErrorMessageNamesClass() {
+        List<Throwable> causes = causesOf(assertThrows(JsonbException.class,
+                                                       () -> defaultJsonb.fromJson(JSON, BrokenSetterPojo.class)));
+
+        String expectedMessage = "Error setting value on: " + BrokenSetterPojo.class.getName();
+        assertTrue(causes.stream().anyMatch(cause -> expectedMessage.equals(cause.getMessage())),
+                   "No exception with the message: " + expectedMessage);
+    }
+
+    private static List<Throwable> causesOf(Throwable throwable) {
+        List<Throwable> causes = new ArrayList<>();
+        for (Throwable cause = throwable; cause != null; cause = cause.getCause()) {
+            causes.add(cause);
+        }
+        return causes;
+    }
+
+    /**
+     * A POJO whose setter throws a {@link RuntimeException}, and whose {@code toString()} must not
+     * be called while it is deserialized.
+     */
+    public static class BrokenSetterPojo {
+
+        public void setIban(String iban) {
+            throw new IllegalArgumentException("simulated setter failure");
+        }
+
+        @Override
+        public String toString() {
+            throw new IllegalStateException("toString() must not be called during deserialization");
+        }
+    }
+}