1. Overview
The HTTP protocol provides comprehensive information about requested web resources. One of its header fields, Content-Length, specifies the size of a resource in bytes. We can extract this information using the URLConnection class.
Knowing the size of a web file before downloading helps estimate the amount of network data required to download the resource.
In this tutorial, we’ll explore how to get the size of a web file using the getContentLengthLong() and getHeaderField() methods of the URLConnection class.
2. HTTP Content-Length
The Content-Length attribute specifies the size of a web file in the HTTP headers. Since HTTP is a transfer protocol, it provides details of an incoming response. Simply put, the Content-Length field represents the size of the response body in bytes.
Furthermore, the Transfer-Encoding field can be specified and set to chunked instead of the Content-Length field. In this case, we can’t determine the size of the web file as the download is done in chunks.
Notably, the Content-Length header isn’t always accurate. Servers can set this field to any arbitrary value, which may not represent the true file size. For example, when using ResponseEntity in a Spring application, a developer can set the header field to any value, which may not correspond to the actual size of the response body.
3. The getContentLength() and getContentLengthLong() Methods
The URLConnection class in Java provides methods to establish connections with URL resources for write or read operations.
It provides a method named getContentLength() that returns the content length of the HTTP header field as an integer. However, this method can’t represent numbers greater than Integer.MAX_VALUE, which means it cannot handle file sizes greater than 2 GiB.
To address the limitation of the getContentLength() method, the URLConnection class provides the getContentLengthLong() method, which returns the content length as a long value. This method is preferable as it can retrieve large file sizes that exceed Integer.MAX_VALUE.
Notably, the getContentLength() and getContentLengthLong() methods return -1 when the Content-Length field is missing from the HTTP headers.
4. Using the getContentLengthLong() Method
Let’s see an example that retrieves a dummy PDF file size from “https://www.ingka.com/wp-content/uploads/2020/11/dummy.pdf“.
First, let’s define a URL instance that represents the URL to the web file:
String fileUrl = "https://www.ingka.com/wp-content/uploads/2020/11/dummy.pdf";
URL url = new URL(fileUrl);
Next, let’s create a URLConnection object and open a connection to this URL:
URLConnection urlConnection = url.openConnection();
Then, let’s get the size of the web file:
long fileSize = urlConnection.getContentLengthLong();
if (fileSize != -1) {
assertEquals(29789, fileSize);
} else {
fail("Could not determine file size");
}
In the code above, we invoke the getContentLengthLong() method on the urlConnection object to get an estimated file size. Then, we assert that the estimated file size equals the expected size.
Also, we handle the case where a web file size couldn’t determine the web file size.
5. Using the getHeaderField() Method
Alternatively, we can use the getHeaderField() method to retrieve the web file size. The method returns the name field value it accepts as an argument.
Here’s an example that uses the getHeaderField() method:
@Test
void givenUrl_whenGetFileSizeUsingURLConnectionAndGetHeaderField_thenCorrect() throws IOException {
URL url = new URL(fileUrl);
URLConnection urlConnection = url.openConnection();
String headerField = urlConnection.getHeaderField("Content-Length");
if (headerField != null && !headerField.isEmpty()) {
long fileSize = Long.parseLong(headerField);
assertEquals(29789, fileSize);
} else {
fail("Could not determine file size");
}
}
In the code above, we invoke the getHeaderField() method on the URLConnection object and specify the Content-Length header field. Since the method returns a string, we parse its value to a long type and assert that the return size is equal to the expected size.
6. Conclusion
In this tutorial, we saw how to use the URLConnection.getContentLengthLong() and URLConnection.getHeaderField() methods to retrieve the size of a known web file. We leverage the Content-Length field of the HTTP header to determine the web file size. When the file size can’t be determined, it returns a value of negative one (-1).
As always, the complete source code for the examples is available over on GitHub.