我有一个与rest-assured无关的类似问题，但这是Google发现的第一个结果，所以我在这里发布我的答案，以防其他人面临同样的问题。
对我来说，问题是（正如ConnectionClosedException明确指出的那样）closing在阅读响应之前的连接。沿着如下的东西：

CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);

try {
    doSomthing();
} finally {
    response.close();
}
HttpEntity entity = response.getEntity();
InputStream instream = entity.getContent(); // Response already closed. This won't work!

修复是显而易见的。排列代码，使响应在关闭后不被使用：

CloseableHttpClient httpclient = HttpClients.createDefault();
HttpGet httpget = new HttpGet("http://localhost/");
CloseableHttpResponse response = httpclient.execute(httpget);

try {
    doSomthing();
    HttpEntity entity = response.getEntity();
    InputStream instream = entity.getContent(); // OK
} finally {
    response.close();
}

也许你可以尝试摆弄连接配置？例如：

given().config(RestAssured.config().connectionConfig(connectionConfig().closeIdleConnectionsAfterEachResponse())). ..