HttpHeaders.EMPTY is not immutable [SPR-17633]

[spring-projects-issues]

Alfred Thomas opened SPR-17633 and commented

According to the Javadoc, HttpHeaders.EMPTY is immutable.
I found that by using the following code, headers could be added to HttpHeaders.EMPTY.
From then on, any further calls will result in a non-empty HttpHeaders.

HttpHeaders myHeaders = HttpHeaders.writableHttpHeaders(HttpHeaders.EMPTY);
my.add(HttpHeaders.ACCEPT_ENCODING, "gzip");

I have written a simple unit test to explain this issue:

@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**
}

This caused a bug in my code where I subsequently used HttpHeaders.EMPTY again to create a new writable HttpHeaders object. I then unknowingly introduced headers used in the previous object

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Affects: 5.1.3

Reference URL: https://stackoverflow.com/questions/53982635/possible-bug-in-spring-httpheaders

Issue Links:

• Improve WebFlux performance for header management [SPR-17250] #21783 Improve WebFlux performance for header management

Referenced from: commits 6e3f974