Java http request response headers

In a previous article, I showed you how to automatically add information when logging from a Java Spring application to Application Insights logs using the Mapped Diagnostic Context. The code for that article is here.

The example I used in that article generated a Request Key every time a controller method is calling. An alternative solution is to have the client that calls the controller pass the Request Key to method in the HTTP Header.

Request Headers

To read the Header of an HTTP request in a controller method, add a parameter of type HttpServletRequest.

An HTTP header can contain one or more key/value pairs. The HttpServletRequest object allows you to read the value of a given key, using the getHeader method and pasing in the key. If we add an HTTP Header with the key «X-Request-Key» to our request, we can retrieve it java with a line like:

String request requsestKey = request.getHeader("X-Request-Key");

The modified controller code looks like this:

@GetMapping("add//") public ResponseEntityInteger> Add( @PathVariable("firstNumber") Integer firstNumber, @PathVariable("secondNumber") Integer secondNumber, HttpServletRequest request) < String requestKey = request.getHeader("X-Request-Key"); if (requestKey == null) < requestKey = UUID.randomUUID().toString(); >MDC.put("Request-Key", requestKey); logger.info("MathController.Add() called with " + firstNumber + " and " + secondNumber); Integer sum = mathService.AddNumbers(firstNumber, secondNumber); return new ResponseEntityInteger>(sum, HttpStatus.OK); >

Using a tool like Postman, I can send an HTTP request and add a custom header to that request, as shown in Fig. 1.

Fig. 1

This works the same as the code in my previous article, except the calling client is given the opportunity to pass in the Request Key, rather than having the controller generate it.

Response Headers

An HTTP Response also contains Header information and we can write Java code to include custom key/value pairs in the Response Header.

The HTTPHeaders object (in the org.springframework.http namespace) allows you to write to the response header. This object has a set method that accepts a key and a value as parameters, so you can pass that back with the HTTP Response Headers.

Our existing code returned a ResponseEntity object, which has a constructor overload that allows us to include an HTTPHeaders object in the response.

Here is the updated code in the controller:

@GetMapping("add//") public ResponseEntityInteger> Add( @PathVariable("firstNumber") Integer firstNumber, @PathVariable("secondNumber") Integer secondNumber, HttpServletRequest request) < String requestKey = request.getHeader("X-Request-Key"); if (requestKey == null) < requestKey = UUID.randomUUID().toString(); >MDC.put("Request-Key", requestKey); logger.info("MathController.Add() called with " + firstNumber + " and " + secondNumber); Integer sum = mathService.AddNumbers(firstNumber, secondNumber); HttpHeaders responseHeaders = new HttpHeaders(); responseHeaders.set("X-Request-Key", requestKey); return new ResponseEntityInteger>(sum, responseHeaders, HttpStatus.OK); >

Now, when we run the application and call this controller method, we not only get information in the body of the HTTP Response; we also receive the Request Key in the HTTP Response Header, as shown in Fig. 2.

Fig. 2

Conclusion

In this article, you learned how to read Header information from an HTTP Request and to write Header information to an HTTP Response, using Java code.

You can find the code for this article here.

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Class Headers

HTTP request and response headers are represented by this class which implements the interface Map >. The keys are case-insensitive Strings representing the header names and the value associated with each key is a List with one element for each occurrence of the header name in the request or response.

For example, if a response header instance contains one key «HeaderName» with two values «value1 and value2» then this object is output as two header lines:

HeaderName: value1 HeaderName: value2

getFirst(String) returns a single valued header or the first value of a multi-valued header.
add(String,String) adds the given header value to the list for the given key.
set(String,String) sets the given header field to the single value given overwriting any existing values in the value list.

An instance of Headers is either mutable or immutable. A mutable headers allows to add, remove, or modify header names and values, e.g. the instance returned by HttpExchange.getResponseHeaders() . An immutable headers disallows any modification to header names or values, e.g. the instance returned by HttpExchange.getRequestHeaders() . The mutator methods for an immutable headers instance unconditionally throw UnsupportedOperationException .

All methods in this class reject null values for keys and values. null keys will never be present in HTTP request or response headers.

Nested Class Summary

Nested classes/interfaces declared in interface java.util.Map

Constructor Summary

Method Summary

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

Returns the first value from the List of String values for the given key , or null if no mapping for the key exists.

Methods declared in class java.lang.Object

Methods declared in interface java.util.Map

Constructor Details

Headers

Method Details

size

Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE .

isEmpty

containsKey

Returns true if this map contains a mapping for the specified key. More formally, returns true if and only if this map contains a mapping for a key k such that Objects.equals(key, k) . (There can be at most one such mapping.)

containsValue

Returns true if this map maps one or more keys to the specified value. More formally, returns true if and only if this map contains at least one mapping to a value v such that Objects.equals(value, v) . This operation will probably require time linear in the map size for most implementations of the Map interface.

get

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. More formally, if this map contains a mapping from a key k to a value v such that Objects.equals(key, k) , then this method returns v ; otherwise it returns null . (There can be at most one such mapping.) If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it’s also possible that the map explicitly maps the key to null . The containsKey operation may be used to distinguish these two cases.

getFirst

Returns the first value from the List of String values for the given key , or null if no mapping for the key exists.

put

Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true .)

add

Adds the given value to the list of headers for the given key . If the mapping does not already exist, then it is created.

set

Sets the given value as the sole header value for the given key . If the mapping does not already exist, then it is created.

remove

Removes the mapping for a key from this map if it is present (optional operation). More formally, if this map contains a mapping from key k to value v such that Objects.equals(key, k) , that mapping is removed. (The map can contain at most one such mapping.) Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key. If this map permits null values, then a return value of null does not necessarily indicate that the map contained no mapping for the key; it’s also possible that the map explicitly mapped the key to null . The map will not contain a mapping for the specified key once the call returns.

putAll

Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress.

clear

Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.

keySet

Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove , Set.remove , removeAll , retainAll , and clear operations. It does not support the add or addAll operations.

values

Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove , Collection.remove , removeAll , retainAll and clear operations. It does not support the add or addAll operations.

entrySet

Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove , Set.remove , removeAll , retainAll and clear operations. It does not support the add or addAll operations.

equals

It is reflexive: for any non-null reference value x , x.equals(x) should return true .
It is symmetric: for any non-null reference values x and y , x.equals(y) should return true if and only if y.equals(x) returns true .
It is transitive: for any non-null reference values x , y , and z , if x.equals(y) returns true and y.equals(z) returns true , then x.equals(z) should return true .
It is consistent: for any non-null reference values x and y , multiple invocations of x.equals(y) consistently return true or consistently return false , provided no information used in equals comparisons on the objects is modified.
For any non-null reference value x , x.equals(null) should return false .

An equivalence relation partitions the elements it operates on into equivalence classes; all the members of an equivalence class are equal to each other. Members of an equivalence class are substitutable for each other, at least for some purposes.

hashCode

Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
If two objects are equal according to the equals method, then calling the hashCode method on each of the two objects must produce the same integer result.
It is not required that if two objects are unequal according to the equals method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

of

Returns an immutable Headers with the given name value pairs as its set of headers. The supplied String instances must alternate as header names and header values. To add several values to the same name, the same name must be supplied with each new value. If the supplied headers is empty, then an empty Headers is returned.

of

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