diff --git a/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSource.java b/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSource.java
index 4a2354e180..ce3230fa43 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSource.java
@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.upstream;
 
 import android.net.Uri;
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import java.io.IOException;
 
@@ -79,7 +80,7 @@ public interface DataSource {
    *
    * @return The {@link Uri} from which data is being read, or null if the source is not open.
    */
-  Uri getUri();
+  @Nullable Uri getUri();
 
   /**
    * Closes the source.
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSpec.java b/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSpec.java
index a6b89a334d..ad7a9d0147 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSpec.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/upstream/DataSpec.java
@@ -61,7 +61,7 @@ public final class DataSpec {
   /**
    * Body for a POST request, null otherwise.
    */
-  public final byte[] postBody;
+  public final @Nullable byte[] postBody;
   /**
    * The absolute position of the data in the full stream.
    */
@@ -81,12 +81,12 @@ public final class DataSpec {
    * A key that uniquely identifies the original stream. Used for cache indexing. May be null if the
    * {@link DataSpec} is not intended to be used in conjunction with a cache.
    */
-  @Nullable public final String key;
+  public final @Nullable String key;
   /**
    * Request flags. Currently {@link #FLAG_ALLOW_GZIP} and
    * {@link #FLAG_ALLOW_CACHING_UNKNOWN_LENGTH} are the only supported flags.
    */
-  @Flags public final int flags;
+  public final @Flags int flags;
 
   /**
    * Construct a {@link DataSpec} for the given uri and with {@link #key} set to null.
@@ -128,7 +128,8 @@ public final class DataSpec {
    * @param key {@link #key}.
    * @param flags {@link #flags}.
    */
-  public DataSpec(Uri uri, long absoluteStreamPosition, long length, String key, @Flags int flags) {
+  public DataSpec(
+      Uri uri, long absoluteStreamPosition, long length, @Nullable String key, @Flags int flags) {
     this(uri, absoluteStreamPosition, absoluteStreamPosition, length, key, flags);
   }
 
@@ -143,7 +144,12 @@ public final class DataSpec {
    * @param key {@link #key}.
    * @param flags {@link #flags}.
    */
-  public DataSpec(Uri uri, long absoluteStreamPosition, long position, long length, String key,
+  public DataSpec(
+      Uri uri,
+      long absoluteStreamPosition,
+      long position,
+      long length,
+      @Nullable String key,
       @Flags int flags) {
     this(uri, null, absoluteStreamPosition, position, length, key, flags);
   }
@@ -162,7 +168,7 @@ public final class DataSpec {
    */
   public DataSpec(
       Uri uri,
-      byte[] postBody,
+      @Nullable byte[] postBody,
       long absoluteStreamPosition,
       long position,
       long length,
@@ -222,4 +228,13 @@ public final class DataSpec {
     }
   }
 
+  /**
+   * Returns a copy of this {@link DataSpec} with the specified Uri.
+   *
+   * @param uri The new source {@link Uri}.
+   * @return The copied {@link DataSpec} with the specified Uri.
+   */
+  public DataSpec withUri(Uri uri) {
+    return new DataSpec(uri, postBody, absoluteStreamPosition, position, length, key, flags);
+  }
 }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/upstream/PriorityDataSource.java b/library/core/src/main/java/com/google/android/exoplayer2/upstream/PriorityDataSource.java
index a36ccd11b1..729f7fe179 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/upstream/PriorityDataSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/upstream/PriorityDataSource.java
@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.upstream;
 
 import android.net.Uri;
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.PriorityTaskManager;
 import java.io.IOException;
@@ -63,7 +64,7 @@ public final class PriorityDataSource implements DataSource {
   }
 
   @Override
-  public Uri getUri() {
+  public @Nullable Uri getUri() {
     return upstream.getUri();
   }
 
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/util/UriUtil.java b/library/core/src/main/java/com/google/android/exoplayer2/util/UriUtil.java
index 6592273d03..071ebf2084 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/util/UriUtil.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/util/UriUtil.java
@@ -143,6 +143,26 @@ public final class UriUtil {
     }
   }
 
+  /**
+   * Removes query parameter from an Uri, if present.
+   *
+   * @param uri The uri.
+   * @param queryParameterName The name of the query parameter.
+   * @return The uri without the query parameter.
+   */
+  public static Uri removeQueryParameter(Uri uri, String queryParameterName) {
+    Uri.Builder builder = uri.buildUpon();
+    builder.clearQuery();
+    for (String key : uri.getQueryParameterNames()) {
+      if (!key.equals(queryParameterName)) {
+        for (String value : uri.getQueryParameters(key)) {
+          builder.appendQueryParameter(key, value);
+        }
+      }
+    }
+    return builder.build();
+  }
+
   /**
    * Removes dot segments from the path of a URI.
    *
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/util/UriUtilTest.java b/library/core/src/test/java/com/google/android/exoplayer2/util/UriUtilTest.java
index a52867e1b2..82c62ecb3e 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/util/UriUtilTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/util/UriUtilTest.java
@@ -15,9 +15,11 @@
  */
 package com.google.android.exoplayer2.util;
 
+import static com.google.android.exoplayer2.util.UriUtil.removeQueryParameter;
 import static com.google.android.exoplayer2.util.UriUtil.resolve;
 import static com.google.common.truth.Truth.assertThat;
 
+import android.net.Uri;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RobolectricTestRunner;
@@ -104,4 +106,36 @@ public final class UriUtilTest {
     assertThat(resolve("a:b", "../c")).isEqualTo("a:c");
   }
 
+  @Test
+  public void removeOnlyQueryParameter() {
+    Uri uri = Uri.parse("http://uri?query=value");
+    assertThat(removeQueryParameter(uri, "query").toString()).isEqualTo("http://uri");
+  }
+
+  @Test
+  public void removeFirstQueryParameter() {
+    Uri uri = Uri.parse("http://uri?query=value&second=value2");
+    assertThat(removeQueryParameter(uri, "query").toString()).isEqualTo("http://uri?second=value2");
+  }
+
+  @Test
+  public void removeMiddleQueryParameter() {
+    Uri uri = Uri.parse("http://uri?first=value1&query=value&last=value2");
+    assertThat(removeQueryParameter(uri, "query").toString())
+        .isEqualTo("http://uri?first=value1&last=value2");
+  }
+
+  @Test
+  public void removeLastQueryParameter() {
+    Uri uri = Uri.parse("http://uri?first=value1&query=value");
+    assertThat(removeQueryParameter(uri, "query").toString()).isEqualTo("http://uri?first=value1");
+  }
+
+  @Test
+  public void removeNonExistentQueryParameter() {
+    Uri uri = Uri.parse("http://uri");
+    assertThat(removeQueryParameter(uri, "foo").toString()).isEqualTo("http://uri");
+    uri = Uri.parse("http://uri?query=value");
+    assertThat(removeQueryParameter(uri, "foo").toString()).isEqualTo("http://uri?query=value");
+  }
 }
diff --git a/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java b/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java
index 2675e1f0d7..de623b59c9 100644
--- a/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java
+++ b/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java
@@ -217,6 +217,11 @@ public class FakeDataSource implements DataSource {
     return dataSpecs;
   }
 
+  /** Returns whether the data source is currently opened. */
+  public final boolean isOpened() {
+    return opened;
+  }
+
   protected void onDataRead(int bytesRead) throws IOException {
     // Do nothing. Can be overridden.
   }
diff --git a/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java b/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java
index 905adae092..ffc877bf42 100644
--- a/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java
+++ b/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java
@@ -159,6 +159,11 @@ public class FakeMediaSource extends BaseMediaSource {
     }
   }
 
+  /** Asserts that the source has been prepared. */
+  public void assertPrepared() {
+    assertThat(preparedSource).isTrue();
+  }
+
   /**
    * Assert that the source and all periods have been released.
    */