spring - Spring HttpHeaders 中可能存在的错误

Question

我发现我认为可能是 Spring 类中的一个错误 HttpHeaders和 ReadOnlyHttpHeaders .在使用 Spring 提出 Jira 缺陷之前，我想确认这一点。这是我用来创建空 HttpHeaders 的代码片段目的:

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

然后我使用以下方法将标题添加到我的新对象:

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

在此之后 HttpHeaders.EMPTY不再为空

HttpHeaders.EMPTY.size() == 1

HttpHeaders.EMPTY 的 javadoc 指出:

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

这里的问题是，当 'HttpHeaders.EMPTY' 在其他地方使用时，它会引入意外的 header 。

考虑以下单元测试:

@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()); 
}

以下是单元测试的结果:

// 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

我觉得这是一个错误，因为 HttpHeaders.EMPTY应该是不可变的。
我还可以通过在 Spring HttpHeaders.java 中进行两项更改来解决此问题和 ReadOnlyHttpHeaders.java

Answer 1

是的，你是对的，这可能是 spring 框架的错误 HttpHeaders

public static final HttpHeaders EMPTY

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

案例:1
一起来看看 HttpHeaders.Empty ，它返回不可变对象(immutable对象)

    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

结论:

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

案例:2 从 writableHttpHeaders 返回对象反射(reflect)到 HttpHeaders.EMPTY单例对象(内部和间接)

    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
    

结论:

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

案例:3 假设如果我们将 HttpHeaders 对象的空实例传递给 writableHttpHeaders使用构造函数就没有问题，一切都很清楚

    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

案例:4 即使间接不可变 HttPHeaders.EMPTY可以修改，但是直接修改还是会报错

    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");

输出:

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)

最终结论:是的，这是错误，您可以为 Spring 项目提出错误 spring-bug , 不可变对象(immutable对象)不能改变状态