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Why should i use url.openStream instead of of url.getContent?

2014-10-18 14:09 190 查看

I would like to retrieve the content of a url. Similar to pythons:

html_content = urllib.urlopen("http://www.test.com/test.html").read()

In examples( java2s.com ) you see very often the following code:

URL url = new URL("http://www.test.com/test.html");
String foo = (String) url.getContent();

The Description of getContent is the following:

Gets the contents of this URL. This method is a shorthand for: openConnection().getContent()
Returns: the contents of this URL.

In my opinion that should work perfectly fine. Buuut obviously this code doesnt work, because it raises an error:

Exception in thread "main" java.lang.ClassCastException: sun.net.www.protocol.http.HttpURLConnection$HttpInputStream cannot be cast to java.lang.String

Obviously it returns an inputStream.

So i ask myself: what's the purpose of this function which isn't doing what it is seems to do? And why is no hint for quirks it in the documentation? And why did i saw it in several examples?

Or am i getting this wrong?

The suggested solution (stackoverflow) is to use url.openStream() and then read the Stream.

As you said, documentation says that

URL.getContent()

is a shortcut for

openConnection().getContent()

so we need to look at the documentation for

URLConnection.getContent()

.

We can see that this returns an

Object

the type of which is determined by the the

content-type

header field of the response. This type determines the

ContentHandler

that will be used. So a

ContentHandler

converts data based on its MIME type to the appropriate class of Java Object.

In other words the type of Object you get will depend on the content served. For example, it wouldn't make sense to return a

String

if the MIME type was

image/png

.

This is why in the example code you link to at java2s.com they check the class of the returned Object:

try {
URL u = new URL("http://www.java2s.com");
Object o = u.getContent();
System.out.println("I got a " + o.getClass().getName());
} catch (Exception ex) {
System.err.println(ex);
}

So you can say

String foo = (String) url.getContent();

if you know your

ContentHandler

will return a

String

.

There are default content handlers defined in the

sun.net.www.content

package but as you can see they are returning streams for you.

You could create your own

ContentHandler

that does return a

String

but it will probably be easier just to read the Stream as you suggest.

        URL url = new URL("http://www.so.com");
URLConnection.setContentHandlerFactory(new ContentHandlerFactory() {
@Override
public ContentHandlercreateContentHandler(String mimetype) {
return new ContentHandler() {

@Override
public Object getContent(URLConnection urlc) throws IOException {
InputStream input = urlc.getInputStream();
StringBuffer stringBuffer = new StringBuffer();
byte[] bytes = new byte[1024];
while(input.read() != -1){
input.read(bytes);
stringBuffer.append(new String(bytes));

}
return stringBuffer.toString();
}
};
}
});
String str = (String)url.getContent();
System.out.println(str);
/*
byte[] bytes = new byte[1024];
InputStream input = (InputStream)url.getContent();
StringBuffer stringBuffer = new StringBuffer();
while(input.read() != -1){
input.read(bytes);
stringBuffer.append(new String(bytes));

}

System.out.println(stringBuffer.toString());
*/

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