You are missing json-simple-1.1.1.jar from your classpath.

if you are using Maven, add below into your pom.xml.

<dependency>
    <groupId>com.googlecode.json-simple</groupId>
    <artifactId>json-simple</artifactId>
    <version>1.1.1</version>
</dependency>

Or you can download it from here.

Use getJSONObject method

JSONObject innerObj =  jsonArray.getJSONObject(0);

Returns the value at index if it exists and is a JSONObject.

Packageless classes are invisible for classes inside a package. Since JSP's are compiled into a class in a servletcontainer-specific package, the packageless JSONCore class is invisible for the JSP.

Put the JSONCore class in a package and use either the full qualified class name to declare and instantiate it, or use the @page import statement.

That said, the JSP is the wrong place for the job. You should be using (indirectly) a Servlet class for the job. See also How to avoid Java code in JSP files.

Probably your simple json.jar file isn't in your classpath.

I thought this was something that came with java and an external jar is not needed

No - JsonObject is only in JavaEE. JSONObject isn't included in JavaSE, it needs an external library. You can find it on github. However, if you use JavaEE, it is included.

If you are using gradle, add the following to your build.gradle file (available in maven central):

dependencies {
    compile "org.json:json:20171018"
}

You did not share the most import bits of the code. I'm guessing the type T passed to the generic method is a single type while the response is an array.

I saw other people in forums using at same way JsonArray.

For sure they are not using JsonArray from the javax.json package. They might be using org.json.JSONArray or com.google.gson.JsonArray or any other class.

Why I get this message?

javax.json.JsonArray is an interface and interfaces cannot be instantiated.

Here are a few examples quoted from the JsonArray API documentation that demonstrate how to use it:

The following example demonstrates how to create a JsonArray object from an input source using the method JsonReader.readArray():

JsonReader jsonReader = Json.createReader(...);
JsonArray array = jsonReader.readArray();
jsonReader.close();

The following example demonstrates how to build an empty JSON array using the class JsonArrayBuilder:

JsonArray array = Json.createArrayBuilder().build();

The example code below demonstrates how to create the following JSON array:

[
  {
    "type": "home",
    "number": "212 555-1234"
  },
  {
    "type": "fax",
    "number": "646 555-4567"
  }
]

JsonArray value = Json.createArrayBuilder()
    .add(Json.createObjectBuilder()
        .add("type", "home")
        .add("number", "212 555-1234"))
    .add(Json.createObjectBuilder()
        .add("type", "fax")
        .add("number", "646 555-4567"))
    .build();