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Redirect All Request from web server to another

I have to make an application in tomcat which core responsibility is to redirect all request coming from browser application to API server. It works like proxy server between browser application and API application. Have to write it on tomcat. How can i achieve this, do i have write a REST API application on tomcat too?
Project Description
Requirement is like we have two application one is developed on React, its a web-portal and the other one is on spring-boot as a backend application (json apis).
spring-boot application is going to be deployed on secure zone and web-portal is in DMZ. web-portal in on internet, backend application is on intranet. As web-portal is a CSR application so the api's calls from web-portal contains backend application IP/URI but it cannot be accessible from internet.
So i am planning to write an application on DMZ zone and it will be deployed on apache tomcat, so like when request comes at tomcat it redirect it to backend application server which is on intranet with all headers/payload and redirect response from backend server to web-portal.

You want to set up a reverse proxy which is a standard pattern. Take a look at the answers at Reverse Proxy Java or just search the web for "reverse proxy java".
If you are not bound to implement it in java take a look at Nginx which we use in many projects to transfer the request from the web app to the REST backend.

consuming a java soap wep service using php web application

I'm building a soap web service using Java for bank payment methods, and I'd like to know how can I use or consume this web service in another web application create with php as a e-commerce web site to use these methods for payment.

There are three method to consuming web service (Soap) using php client, please refer to the below link:
Consuming web service (Soap) using php client

how to communicate android application to mysql database through JSP and Servlet

I am creating an android application and I want that when user fill up the registration form in my android application then that data should be saved in remote MySQL database.But i want JSP and Servlet for this as middle level.
I searched a lot on internet but every tutorial shown on internet are using PHP. I don't want to use PHP at server side.
So is there any tutorial or resource for saving android application data to remote MySQL database using JSP and Servlet.

You usually call RESTful services from the Android App. The RESTful services can be written in any language of your choice. Your Android client is one which is communicating with the REST Backend. If you are using ButtonView and on the click of it some update or retrieval are supposed to be performed then REST based services are best.
I would recommend this approach.

Look at the way servlet is deployed from the below blog :
http://hmkcode.com/java-servlet-send-receive-json-using-jquery-ajax/
Hit the servlet URL in a browser.If u get the JSON response, the same call can be leveraged from the mobile client building the URLconnection and rendered in the app using some Parser at Mobile end.
Hope this meets the requirement.

Does iPad development work with interacting through Java code with Spring Framework using TomCat to host the web service on a server? How?

Here is the background of my situation:
I want to create an iPad application that interacts with a oracle SQL database. I have existing Java code from my Flex application that handles all the database requests, and modifications using the Spring Framework. The Flex Application ran as a web service through TomCat. Now I want to make that flex application into a mobile iPad version. I am having trouble figuring out what is the easiest way to use existing Java code and use it for the iPad because the iPad interacts using URL requests instead of direct with the Java.
My question is, can I use the existing Java code with the Spring framework to save time from coding all the back-end handling? Basically I want to access all the classes from my Java code by doing Requests from the iPad. Is this possible and will I need JSON or XML to interact between the iPad and the Java code?
Summary:
Can I use
iPad Objective-C <-----> Java (with spring framework) on TomCat Web Service to handle oracle SQL data handeling? If so, how and what technologies do I need? Will I need JSON or XML and how does that factor between the iPad and Java?
Thanks!

A good approach would be to design your app to communicate with RESTful services that return JSON. Once this is done your iPad app doesn't have to even know that the server code is written in Java.. it's just interacting over HTTP.
Here's a good tutorial on setting up your tomcat to host your RESTful services: http://www.vogella.com/articles/REST/article.html - I've used this for an app I'm developing. Spring isn't even necessary.
You could go XML, but JSON is just easier in my opinion. Here's a good blog outlining the good and bad of both sides. http://digitalbazaar.com/2010/11/22/json-vs-xml/

OK, I'm making the following presumptions.
Your flex application runs on a different machine from the Tomcat
server
Your flex application makes web service calls to the Tomcat server
So, the flex application doesn't know the underlying technology that provides the web services. It's just seeing/consuming the output
There's no reason why the iPad app can't do the same thing. There's no reason why it can't use the same web services that the Flex application uses. It could consume the same messages (Assuming it can handle the request/response format currently employed by the Flex application).
You can make changes if you like if you want to change the structure of the requests/responses between the clients. But the clients don't know (nor care) how the web services are implemented. They are just requesting and consuming info.

Java application vs web service vs web application

I am developing a multi-platform (Android, iPhone, Windows and Blacbberry) mobile application. The application needs to communicate with our server for several tasks, such as retrieving buddy lists etc. The server interacts with data that is stored in a MySQL database. I intend to code the server element in Java, however I am confused by all the different types. So far, I think I have narrowed it down to three options:
1) I code the application using Jetty to accept http posts. I post XML to the server, handle it, interact with the DB and post a XML response back. I would save the application as a jar and leave it running on my server.
2)I develop a Java web service. REST/JSON/SOAP?
3)I develop a Java web application.
Whilst there are many questions already out there asking what the differences is, I am struggling to find a clear explanation as to what is the best approach in which situation. I have previously used the first approach but am assuming the second approach is the better option, I'm just not sure what the advantage is.

your 1-3 options are all variants of a "Web application".
Jetty is a Java based http server/servlet container. If you want to communicate between client and server using http, you are using an http server (although not necessarily Jetty).
A Web Service is part of a web application that conforms to a standard around how clients communicate with the server, and how the server offers up information to the clients.
A web application is a Java application that makes it services available over http.
So if you want to have your clients communicate with a server, and store info in a db, you are using a web-application.

I would recommend going with option 2 as it is more lightweight and can be parsed directly in you're web application. XML got more overhead and must be translated, while you can just serialize objects directly to JSON from you're Java application and then parse them in javascript at frontend