AWS Can not deserialize instance of java.lang.String out of START_OBJECT

Question

I have made a Lambda Function and I want to access it via URL with a help of the API Gateway.

I have set it all up, I have also created an application/json body mapping template in API Gateway looking like this:

{      "input": "$input.params('input')", } 

And then I am triggering HTTP GET request that looks like this:

https://dmquh95ckh.execute-api.eu-west-1.amazonaws.com/prod/OtoTestFunction?input=test 

My Java handler class looks like this:

public class LambdaFunctionHandler implements RequestHandler<String, String> {      @Override     public String handleRequest(String input, Context context) {         context.getLogger().log("Input: " + input);         return "Test completed."+input;     } } 

And this is the full error message:

{   "errorMessage": "An error occurred during JSON parsing",   "errorType": "java.lang.RuntimeException",   "stackTrace": [],   "cause": {     "errorMessage": "com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token\n at [Source: lambdainternal.util.NativeMemoryAsInputStream@68c4039c; line: 1, column: 1]",     "errorType": "java.io.UncheckedIOException",     "stackTrace": [],     "cause": {       "errorMessage": "Can not deserialize instance of java.lang.String out of START_OBJECT token\n at [Source: lambdainternal.util.NativeMemoryAsInputStream@68c4039c; line: 1, column: 1]",       "errorType": "com.fasterxml.jackson.databind.JsonMappingException",       "stackTrace": [         "com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)",         "com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:835)",         "com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:59)",         "com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:12)",         "com.fasterxml.jackson.databind.ObjectReader._bindAndClose(ObjectReader.java:1441)",         "com.fasterxml.jackson.databind.ObjectReader.readValue(ObjectReader.java:1047)"       ]     }   } } 

Answer 1

It worked for me in all the scenarios when I change the type of input argument from String to Object.

public class LambdaFunctionHandler implements RequestHandler<Object, String> {    @Override   public String handleRequest(Object input, Context context) {     String data= input != null ? input.toString() : "{}";     context.getLogger().log("Input: " + data);     return "Test completed."+data;   } } 

************************** Added on 12 March 2021 ***************************

After working on a couple of Lambda implementations, I realized that the input argument is nothing but a plain string representation of a JSON structure or a Map<String, Object> representation. For the map representation, Key is the name of the attribute, and the value is (1) a String if it is a primitive value or (2) a List if it has multiple values, is another Map<String, Object> or another JSON structure. You can recover the JSON representation with:

    if(input instanceof String)     {         String lambdaInputJsonStr = (String)input;     }     else if(input instanceof Map)     {         String lambdaInputJsonStr = gson.toJson((Map)input);     }