Avoid infinite loop in AbstractResource#contentLength

Due to changes made in commit 2fa87a71 for SPR-9118,
AbstractResource#contentLength would fall into an infinite loop unless
the method were overridden by a subclass (which it is in the majority of
use cases).

This commit:

 - fixes the infinite recursion by refactoring to a while loop

 - asserts that the value returned from #getInputStream is not null in
   order to avoid NullPointerException

 - tests both of the above

 - adds Javadoc to the Resource interface to clearly document that the
   contract for any implementation is that #getInputStream must not
   return null

Issue: SPR-9161

spring-core/src/main/java/org/springframework/core/io/AbstractResource.java

@@ -25,6 +25,7 @@ import java.net.URISyntaxException;
 import java.net.URL;
 
 import org.springframework.core.NestedIOException;
+import org.springframework.util.Assert;
 import org.springframework.util.ResourceUtils;
 
 /**
@@ -112,13 +113,16 @@ public abstract class AbstractResource implements Resource {
 	 * content length. Subclasses will almost always be able to provide
 	 * a more optimal version of this, e.g. checking a File length.
 	 * @see #getInputStream()
+	 * @throws IllegalStateException if {@link #getInputStream()} returns null.
 	 */
 	public long contentLength() throws IOException {
-		InputStream is = getInputStream();
+		InputStream is = this.getInputStream();
+		Assert.state(is != null, "resource input stream must not be null");
 		try {
 			long size = 0;
 			byte[] buf = new byte[255];
-			for (int read = is.read(buf); read != -1;) {
+			int read;
+			while((read = is.read(buf)) != -1) {
 				size += read;
 			}
 			return size;

spring-core/src/main/java/org/springframework/core/io/Resource.java

@@ -18,6 +18,7 @@ package org.springframework.core.io;
 
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
 import java.net.URI;
 import java.net.URL;
 
@@ -132,4 +133,9 @@ public interface Resource extends InputStreamSource {
 	 */
 	String getDescription();
 
+	/**
+	 * {@inheritDoc}
+	 * @return the input stream for the underlying resource (must not be {@code null}).
+	 */
+	public InputStream getInputStream() throws IOException;
 }