[SalesForce] Create and Link Document to multiple records using rest api

This is possible in java. I tried your code and it is working. As per my observation only thing which could be culprit here is the uri. Below is the code snippet where I have mentioned uri directly.

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.methods.PostMethod;
import org.apache.commons.httpclient.methods.StringRequestEntity;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;


public class Main {

        public static void main(String[] args) throws FileNotFoundException, IOException, ParseException {
            JSONParser parser = new JSONParser();

               // Parser parsing  JSON File having multiple records
               Object obj = parser.parse(new FileReader("JsonData.json"));

               HttpClient httpclient = new HttpClient();
               PostMethod mypost = new PostMethod("https://kmforce-dev-ed.my.salesforce.com/services/data/v34.0/composite/tree/Account/");
               String jsonText = obj.toString();
               System.out.println("\n_JSON TEXT=" + jsonText);
               mypost.setRequestHeader("Authorization", "OAuth " +"<myssessionid>"); 
               mypost.setRequestEntity(new StringRequestEntity(jsonText, "application/json", "UTF-8"));
               int response=httpclient.executeMethod(mypost);
               System.out.println(response);
               System.out.println("Response-->"+mypost.getResponseBodyAsString());
        }
}

Below is the output:

{
    "hasErrors": false,
    "results": [
        {
            "referenceId": "ref1",
            "id": "0019000001bslYJAAY"
        },
        {
            "referenceId": "ref2",
            "id": "0019000001bslYKAAY"
        },
        {
            "referenceId": "ref3",
            "id": "0019000001bslYLAAY"
        },
        {
            "referenceId": "ref4",
            "id": "0019000001bslYMAAY"
        }
    ]
}

Let me know if you face any issue here.

You can definitely use workbench to execute these

Use the below for the request

/services/data/v34.0/composite/tree/Account

and in the body use the below JSON

{
"records" :[{
"attributes" : {"type" : "Account", "referenceId" : "ref1"},
"name" : "SampleAccount1",
"phone" : "1111111111",
"website" : "www.salesforce1.com",
"numberOfEmployees" : "100",
"industry" : "Banking"   
},{
"attributes" : {"type" : "Account", "referenceId" : "ref2"},
"name" : "SampleAccount2",
"phone" : "2222222222",
"website" : "www.salesforce2.com",
"numberOfEmployees" : "250",
"industry" : "Banking"
},{
"attributes" : {"type" : "Account", "referenceId" : "ref3"},
"name" : "SampleAccount3",
"phone" : "3333333333",
"website" : "www.salesforce3.com",
"numberOfEmployees" : "52000",
"industry" : "Banking"
},{
 "attributes" : {"type" : "Account", "referenceId" : "ref4"},
 "name" : "SampleAccount4",
 "phone" : "4444444444",
 "website" : "www.salesforce4.com",
 "numberOfEmployees" : "2500",
 "industry" : "Banking"
  }]
}

You should see the response in workbench

Alternatively you can use POSTMAN to experiment with this API .

You will need to create a connected app in salesforce and use oauth 2.0 to get access token for subsequent calls

use the below link for reference