feat: Allow accepting a JSON substring using a string instead of throwing an exception.

src/main/java/com/fasterxml/jackson/databind/DeserializationFeature.java

@@ -452,6 +452,16 @@ public enum DeserializationFeature implements ConfigFeature
      */
     ACCEPT_FLOAT_AS_INT(true),
 
+    /**
+     * Feature that allow accepting a JSON substring using a string
+     * instead of throwing an exception.
+     *<p>
+     * Feature is enabled by default.
+     *
+     * @since 2.20.0
+     */
+    ACCEPT_SUB_JSON_AS_STRING(false),
+
     /**
      * Feature that determines standard deserialization mechanism used for
      * Enum values: if enabled, Enums are assumed to have been serialized  using

src/main/java/com/fasterxml/jackson/databind/deser/std/StringCollectionDeserializer.java

@@ -1,14 +1,14 @@
 package com.fasterxml.jackson.databind.deser.std;
 
-import java.io.IOException;
-import java.util.*;
-
 import com.fasterxml.jackson.annotation.JsonFormat;
-
 import com.fasterxml.jackson.core.JsonParser;
 import com.fasterxml.jackson.core.JsonToken;
-
-import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.BeanProperty;
+import com.fasterxml.jackson.databind.DeserializationContext;
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.JavaType;
+import com.fasterxml.jackson.databind.JsonDeserializer;
+import com.fasterxml.jackson.databind.JsonMappingException;
 import com.fasterxml.jackson.databind.annotation.JacksonStdImpl;
 import com.fasterxml.jackson.databind.cfg.CoercionAction;
 import com.fasterxml.jackson.databind.cfg.CoercionInputShape;
@@ -19,6 +19,12 @@
 import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
 import com.fasterxml.jackson.databind.type.LogicalType;
 
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Objects;
+
 /**
  * Specifically optimized version for {@link java.util.Collection}s
  * that contain String values; reason is that this is a very common
@@ -197,6 +203,14 @@ public Collection<String> deserialize(JsonParser p, DeserializationContext ctxt,
         if (_valueDeserializer != null) {
             return deserializeUsingCustom(p, ctxt, result, _valueDeserializer);
         }
+
+        if (ctxt.isEnabled(DeserializationFeature.ACCEPT_SUB_JSON_AS_STRING)) {
+            JsonToken currentToken = p.currentToken();
+            if (currentToken == JsonToken.START_OBJECT || currentToken == JsonToken.START_ARRAY) {
+                return deserializeUsingCustom(p, ctxt, result, StringDeserializer.instance);
+            }
+        }
+
         try {
             while (true) {
                 // First the common case:

src/main/java/com/fasterxml/jackson/databind/deser/std/StringDeserializer.java

@@ -1,13 +1,18 @@
 package com.fasterxml.jackson.databind.deser.std;
 
-import java.io.IOException;
-
-import com.fasterxml.jackson.core.*;
-import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.core.JsonToken;
+import com.fasterxml.jackson.databind.DeserializationContext;
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.JsonMappingException;
 import com.fasterxml.jackson.databind.annotation.JacksonStdImpl;
 import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
 import com.fasterxml.jackson.databind.type.LogicalType;
 
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.Deque;
+
 @JacksonStdImpl
 public class StringDeserializer extends StdScalarDeserializer<String> // non-final since 2.9
 {
@@ -36,6 +41,81 @@ public Object getEmptyValue(DeserializationContext ctxt) throws JsonMappingExcep
 
     @Override
     public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException
+    {
+        // disabled, execute default serialization
+        if (!ctxt.isEnabled(DeserializationFeature.ACCEPT_SUB_JSON_AS_STRING)) {
+            return defaultDeserialize(p, ctxt);
+        }
+
+        JsonToken currentToken = p.getCurrentToken();
+
+        // not a JSON substring, execute default serialization
+        if (currentToken != JsonToken.START_OBJECT && currentToken != JsonToken.START_ARRAY) {
+            return defaultDeserialize(p, ctxt);
+        }
+
+        StringBuilder builder = new StringBuilder();
+        Deque<JsonToken> stack = new ArrayDeque<>();
+
+        builder.append(p.getText());
+        stack.push(currentToken);
+
+        final boolean isArray = currentToken == JsonToken.START_ARRAY;
+        while (!stack.isEmpty()) {
+            // an empty stack indicates that the current sub JSON string has been searched and completed
+            JsonToken nextToken = p.nextToken();
+            if (isArray && nextToken == JsonToken.END_ARRAY ||
+                    !isArray && nextToken == JsonToken.END_OBJECT) {
+                stack.pop();
+            }
+            if (isArray && nextToken == JsonToken.START_ARRAY ||
+                    !isArray && nextToken == JsonToken.START_OBJECT) {
+                stack.push(nextToken);
+            }
+
+            // start the sub JSON string, add comma if necessary
+            if (nextToken.isStructStart()) {
+                appendCommaIfNecessary(builder).append(p.getText());
+            }
+
+            // end of sub JSON string, delete comma if necessary
+            else if (nextToken.isStructEnd()) {
+                deleteCommaIfNecessary(builder).append(p.getText());
+            }
+
+            // number, Boolean type, without double quotation marks
+            else if (nextToken.isNumeric() || nextToken.isBoolean()) {
+                builder.append(p.getText());
+            }
+
+            // other types automatically add double quotation marks
+            else {
+                appendCommaIfNecessary(builder).append('"').append(p.getText()).append('"');
+            }
+
+            // automatically add colon if field
+            if (nextToken == JsonToken.FIELD_NAME) {
+                builder.append(':');
+            }
+            // automatically add commas if value
+            else if (nextToken.isScalarValue()) {
+                builder.append(',');
+            }
+        }
+
+        return builder.toString();
+    }
+
+    // Since we can never have type info ("natural type"; String, Boolean, Integer, Double):
+    // (is it an error to even call this version?)
+    @Override
+    public String deserializeWithType(JsonParser p, DeserializationContext ctxt,
+            TypeDeserializer typeDeserializer) throws IOException {
+        return deserialize(p, ctxt);
+    }
+
+    protected String defaultDeserialize(JsonParser p,
+                                        DeserializationContext ctxt) throws IOException
     {
         // The critical path: ensure we handle the common case first.
         if (p.hasToken(JsonToken.VALUE_STRING)) {
@@ -48,11 +128,19 @@ public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOEx
         return _parseString(p, ctxt, this);
     }
 
-    // Since we can never have type info ("natural type"; String, Boolean, Integer, Double):
-    // (is it an error to even call this version?)
-    @Override
-    public String deserializeWithType(JsonParser p, DeserializationContext ctxt,
-            TypeDeserializer typeDeserializer) throws IOException {
-        return deserialize(p, ctxt);
+    private static StringBuilder appendCommaIfNecessary(StringBuilder builder) {
+        char lastChar = builder.charAt(builder.length() - 1);
+        if (lastChar != '{' && lastChar != '[' && lastChar != ':' && lastChar != ',') {
+            builder.append(',');
+        }
+        return builder;
+    }
+
+    private static StringBuilder deleteCommaIfNecessary(StringBuilder builder) {
+        int lastIndex = builder.length() - 1;
+        if (builder.charAt(lastIndex) == ',') {
+            builder.deleteCharAt(lastIndex);
+        }
+        return builder;
     }
 }

src/test/java/com/fasterxml/jackson/databind/StringDeserializerTest.java

@@ -0,0 +1,68 @@
+package com.fasterxml.jackson.databind;
+
+import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+/**
+ * Test validation uses a string to accept JSON substrings
+ * instead of throwing exceptions by default
+ */
+public class StringDeserializerTest
+{
+
+    @Test
+    public void acceptSubJsonTest() throws Exception {
+        String json = "{\"name\":\"root\"," +
+                "\"child\":{\"name\":\"child\"}," +
+                "\"children\":[{\"name\":\"children\"}]," +
+                "\"childrenList\":[{\"name\":\"childrenList\"}]}";
+        ObjectMapper mapper = DatabindTestUtil.newJsonMapper()
+                .configure(DeserializationFeature.ACCEPT_SUB_JSON_AS_STRING, true);
+        TestPojo testPojo = mapper.readValue(json, TestPojo.class);
+        Assertions.assertEquals(testPojo.getChild(), "{\"name\":\"child\"}");
+        Assertions.assertEquals(testPojo.getChildren(), "[{\"name\":\"children\"}]");
+        Assertions.assertEquals(testPojo.getChildrenList().get(0), "{\"name\":\"childrenList\"}");
+    }
+
+    static class TestPojo {
+        private String name;
+        private String child;
+        private String children;
+        private List<String> childrenList;
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+        public String getChild() {
+            return child;
+        }
+
+        public void setChild(String child) {
+            this.child = child;
+        }
+
+        public String getChildren() {
+            return children;
+        }
+
+        public void setChildren(String children) {
+            this.children = children;
+        }
+
+        public List<String> getChildrenList() {
+            return childrenList;
+        }
+
+        public void setChildrenList(List<String> childrenList) {
+            this.childrenList = childrenList;
+        }
+    }
+}

src/test/java/com/fasterxml/jackson/databind/cfg/DeserializationConfigTest.java

@@ -1,17 +1,27 @@
 package com.fasterxml.jackson.databind.cfg;
 
-import java.util.Collections;
-
-import org.junit.jupiter.api.Test;
-
 import com.fasterxml.jackson.annotation.JsonInclude;
-
 import com.fasterxml.jackson.core.json.JsonReadFeature;
-import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.AnnotationIntrospector;
+import com.fasterxml.jackson.databind.DeserializationConfig;
+import com.fasterxml.jackson.databind.DeserializationFeature;
+import com.fasterxml.jackson.databind.MapperFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.PropertyName;
 import com.fasterxml.jackson.databind.introspect.ClassIntrospector;
 import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
 
-import static org.junit.jupiter.api.Assertions.*;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
 
 public class DeserializationConfigTest extends DatabindTestUtil
 {
@@ -101,7 +111,7 @@ public void testEnumIndexes()
         for (DeserializationFeature f : DeserializationFeature.values()) {
             max = Math.max(max, f.ordinal());
         }
-        if (max >= 31) { // 31 is actually ok; 32 not
+        if (max >= 32) { // 32 is actually ok; 33 not
             fail("Max number of DeserializationFeature enums reached: "+max);
         }
     }