如何加Content-Type头部设置为application/json

要在HTTP请求中添加`Content-Type`头部设置为`application/json`，你需要在发送请求时配置合适的请求选项。这里是一些常见编程语言的例子：

### Python (requests库)

import requests

headers = {'Content-Type': 'application/json'}
data = {"expression": "4 + 5 * (2 - 1)"}  # 这里是你的JSON数据
response = requests.post('http://example.com/api/calculation', json=data, headers=headers)

### JavaScript (Fetch API)

const data = { expression: "4 + 5 * (2 - 1)" };
fetch('http://example.com/api/calculation', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  body: JSON.stringify(data)
})
.then(response => response.json())
.then(data => console.log(data.result));

### Java (HttpClient or Apache HttpComponents)

CloseableHttpResponse response;
try (HttpPost httpPost = new HttpPost("http://example.com/api/calculation")) {
    StringEntity entity = new StringEntity("{\"expression\": \"4 + 5 * (2 - 1)\"", ContentType.APPLICATION_JSON);
    httpPost.setEntity(entity);
    httpPost.setHeader(HttpHeaders.CONTENT_TYPE, "application/json");
    response = httpClient.execute(httpPost);
    // ...进一步处理响应
}

在以上代码中，`Content-Type`头部被设置为`application/json`，这样服务器就知道它正在接收的是JSON格式的数据。每个库的具体用法可能会有所不同，但基本思路都是设置一个包含`Content-Type`头的对象，并将其与请求主体关联起来。