Possible bug in Spring HttpHeaders

Question

I found what I believe could be a bug in the Spring classes HttpHeaders and ReadOnlyHttpHeaders. I want to confirm this before raising a Jira defect with Spring. Here is a snippet of the code I use to create an empty HttpHeaders object:

HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(HttpHeaders.EMPTY);

I then add headers to my new object using:

myHeaders.add(HttpHeaders.ACCEPT_ENCODING, "gzip")

After this HttpHeaders.EMPTY is not empty anymore

HttpHeaders.EMPTY.size() == 1

The javadoc for HttpHeaders.EMPTY states:

/**
* The empty {@code HttpHeaders} instance (immutable).
*/
public static final HttpHeaders EMPTY

The problem here is that when 'HttpHeaders.EMPTY' is used elsewhere, it introduces unexpected headers.

Consider the following unit test:

@Test
public void testUpdateEmptyHeaders() {
    assertEquals(0, HttpHeaders.EMPTY.size()); // **Success**
    HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(HttpHeaders.EMPTY);
    myHeaders.add(HttpHeaders.ACCEPT_ENCODING, "gzip");
    assertEquals(0, HttpHeaders.EMPTY.size()); // **Assert Fails**
}

@Test
// This test will fail if run after the test above, but will be successful if run by itself
public void testEmptyHeaders() {
    assertEquals(0, HttpHeaders.EMPTY.size()); 
}

Here is the result of the unit tests:

// testUpdateEmptyHeaders
08:39:28.450 [main] DEBUG org.springframework.test.context.support.AbstractDirtiesContextTestExecutionListener - After test method: context [DefaultTestContext@2e222612, testMethod = testUpdateEmptyHeaders@AuditContextTest, testException = java.lang.AssertionError: expected:<0> but was:<1>

java.lang.AssertionError: 
Expected :0
Actual   :1

// testEmptyHeaders
08:39:28.482 [main] DEBUG org.springframework.test.context.support.AbstractDirtiesContextTestExecutionListener - After test method: context [DefaultTestContext@2e222612, testMethod = testEmptyHeaders@AuditContextTest, testException = java.lang.AssertionError: expected:<0> but was:<1>

java.lang.AssertionError: 
Expected :0
Actual   :1

I feel that this is a bug since HttpHeaders.EMPTY should be immutable. I have also been able to fix this by making two changes in Spring HttpHeaders.java and ReadOnlyHttpHeaders.java

Answer 1

I have reported this bug and it is fixed in Spring 5.1.4 as per https://jira.spring.io/browse/SPR-17633

Answer 2

Yup you are right this might be bug for spring framework HttpHeaders

public static final HttpHeaders EMPTY

HttpHeaders.EMPTY This will return empty HttpHeaders instance (immutable). (and it is singleton)

Case :1 Let's take a look at HttpHeaders.Empty, which returns immutable object

    HttpHeaders head = HttpHeaders.EMPTY;
    
    System.out.println(System.identityHashCode(head));  //1338668845
    
    System.out.println(head.size());                    //0
    
    HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(HttpHeaders.EMPTY); 
    
    System.out.println(System.identityHashCode(myHeaders));  //159413332
    
    myHeaders.add(HttpHeaders.ACCEPT_ENCODING, "gzip"); 
    
    head = HttpHeaders.EMPTY; 
    System.out.println(System.identityHashCode(head));       //1338668845
    System.out.println(head.size());                    //1
    System.out.println(head);                            //{Accept-Encoding=[gzip]}
    
    HttpHeaders head1 = HttpHeaders.EMPTY;
    System.out.println(head1);                          //{Accept-Encoding=[gzip]}
    System.out.println(System.identityHashCode(head1));   //1338668845

Conclusions :

1 : HttpHeaders.EMPTY is always returning the singleton object

2: The problem is when HttpHeaders.EMPTY is passed to writableHttpHeaders method internally the returned object is having relation with HttpHeaders.EMPTY singleton object, look at case 2

Case :2 return object fromwritableHttpHeaders reflects to HttpHeaders.EMPTY singleton object (internally and indirectly)

    HttpHeaders head = HttpHeaders.EMPTY;           
    
    System.out.println(System.identityHashCode(head));  //1338668845
    
    System.out.println(head.size());                //0
    
    HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(HttpHeaders.EMPTY); 
    
    System.out.println(System.identityHashCode(myHeaders));  //159413332
    
    myHeaders.add(HttpHeaders.ACCEPT_ENCODING, "gzip"); 
    
    myHeaders.add("hello", "value");
    
    head = HttpHeaders.EMPTY; 
    System.out.println(System.identityHashCode(head));  //1338668845
    System.out.println(head.size());              //2
    System.out.println(head);                     //{Accept-Encoding=[gzip], hello=[value]}
    
    HttpHeaders head1 = HttpHeaders.EMPTY;
    System.out.println(head1);                    //{Accept-Encoding=[gzip], hello=[value]}
    System.out.println(System.identityHashCode(head1));   //1338668845
    
    myHeaders.remove("hello");
    
    System.out.println(System.identityHashCode(head));     //1338668845
    System.out.println(head.size());                 //1
    System.out.println(head);                       //{Accept-Encoding=[gzip]}
    
    System.out.println(head1);                        //{Accept-Encoding=[gzip]}
    System.out.println(System.identityHashCode(head1));    //1338668845
    

Conclusion :

1 : add and remove operations performed on myHeaders object are reflecting to HttpHeaders.EMPTY object

Case : 3 Suppose if we pass empty instance of the HttpHeaders object to writableHttpHeaders using constructor then there is no issue everything works pretty clear

    HttpHeaders head = HttpHeaders.EMPTY;
    
    System.out.println(System.identityHashCode(head));       //1338668845
    
    System.out.println(head.size());                     //0
    
    HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(new HttpHeaders()); 
    
    System.out.println(System.identityHashCode(myHeaders));       //1323165413
    
    myHeaders.add(HttpHeaders.ACCEPT_ENCODING, "gzip"); 
    
    myHeaders.add("hello", "value");
    
    head = HttpHeaders.EMPTY; 
    System.out.println(System.identityHashCode(head));           //1338668845
    System.out.println(head.size());                    //0
    System.out.println(head);                           //{}
    
    HttpHeaders head1 = HttpHeaders.EMPTY;
    System.out.println(head1);                           //{}
    System.out.println(System.identityHashCode(head1));   //1338668845

Case : 4 Even though indirectly immutable HttPHeaders.EMPTY can be modified, but still it throws an error if you try modifying directly

    HttpHeaders head = HttpHeaders.EMPTY;
    
    System.out.println(System.identityHashCode(head));
    
    System.out.println(head.size());
    
    HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(HttpHeaders.EMPTY); 
    
    System.out.println(System.identityHashCode(myHeaders));
    
    myHeaders.add(HttpHeaders.ACCEPT_ENCODING, "gzip"); 
    
    head = HttpHeaders.EMPTY; 
    System.out.println(System.identityHashCode(head));
    System.out.println(head.size());
    System.out.println(head);
    
    head.add("hello", "value");

Output :

1338668845
0
159413332
1338668845
1
{Accept-Encoding=[gzip]}
Exception in thread "main" java.lang.UnsupportedOperationException
at org.springframework.http.ReadOnlyHttpHeaders.add(ReadOnlyHttpHeaders.java:67)
at com.demo.NestedJsonParse.main(NestedJsonParse.java:40)

Final Conclusion : Yes it is bug you can raise a bug for spring projects spring-bug, immutable objects cannot change the state