Client client = ClientBuilder.newClient();

MultivaluedMap<String, Object> headerMap = new MultivaluedHashMap<String, Object> ();

String username="admin";
String password="admin";
String usernameAndPassword = username + ":" + password;
String authorizationHeaderValue = "Basic " + java.util.Base64.getEncoder().encodeToString( usernameAndPassword.getBytes() );
headerMap.put("Authorization", Arrays.asList(new Object [] { authorizationHeaderValue }));
//headerMap.put("Content-Type", Arrays.asList(new Object [] { "application/json" }));
headerMap.put(HttpHeaders.CONTENT_TYPE, Arrays.asList(new Object [] { MediaType.APPLICATION_JSON }));

JsonObject value = Json.createObjectBuilder()
    .add("var1", var1Object)
    .add("var2", var2Object)
    .add("var3", var3Array)
    .add("var4", var4Array)
    .build();

Response response = client
    .target("https://example.com")
    .request("application/json")
    .headers(headerMap)
    .post(Entity.text(value.toString()));

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.remove(Unknown Source)
at java.util.AbstractList$Itr.remove(Unknown Source)
at java.util.AbstractList.removeRange(Unknown Source)
at java.util.AbstractList.clear(Unknown Source)
at javax.ws.rs.core.AbstractMultivaluedMap.putSingle(AbstractMultivaluedMap.java:97)
at org.glassfish.jersey.message.internal.OutboundMessageContext.setMediaType(OutboundMessageContext.java:708)
at org.glassfish.jersey.client.ClientRequest.type(ClientRequest.java:419)
at org.glassfish.jersey.client.ClientRequest.variant(ClientRequest.java:443)
at org.glassfish.jersey.client.JerseyInvocation$Builder.storeEntity(JerseyInvocation.java:179)
at org.glassfish.jersey.client.JerseyInvocation$Builder.method(JerseyInvocation.java:427)
at org.glassfish.jersey.client.JerseyInvocation$Builder.post(JerseyInvocation.java:334)
at main.java.ofbizrestclientdemo.Main.testCreateProduct(Main.java:438)
at main.java.ofbizrestclientdemo.Main.main(Main.java:520)