diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java
index 0b1b4ba380..25546acb95 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java
@@ -28,14 +28,19 @@ import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.drm.DrmInitData.SchemeData;
 import com.google.android.exoplayer2.drm.ExoMediaDrm.KeyRequest;
 import com.google.android.exoplayer2.drm.ExoMediaDrm.ProvisionRequest;
+import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.EventDispatcher;
 import com.google.android.exoplayer2.util.Log;
+import com.google.android.exoplayer2.util.Util;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.UUID;
+import org.checkerframework.checker.nullness.qual.EnsuresNonNullIf;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
+import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
 /**
  * A {@link DrmSession} that supports playbacks using {@link ExoMediaDrm}.
@@ -84,7 +89,7 @@ import java.util.UUID;
   private final ExoMediaDrm<T> mediaDrm;
   private final ProvisioningManager<T> provisioningManager;
   private final @DefaultDrmSessionManager.Mode int mode;
-  private final HashMap<String, String> optionalKeyRequestParameters;
+  private final @Nullable HashMap<String, String> optionalKeyRequestParameters;
   private final EventDispatcher<DefaultDrmSessionEventListener> eventDispatcher;
   private final int initialDrmRequestRetryCount;
 
@@ -96,13 +101,13 @@ import java.util.UUID;
   private int openCount;
   private HandlerThread requestHandlerThread;
   private PostRequestHandler postRequestHandler;
-  private T mediaCrypto;
-  private DrmSessionException lastException;
-  private byte[] sessionId;
-  private @Nullable byte[] offlineLicenseKeySetId;
+  private @Nullable T mediaCrypto;
+  private @Nullable DrmSessionException lastException;
+  private byte @MonotonicNonNull [] sessionId;
+  private byte @MonotonicNonNull [] offlineLicenseKeySetId;
 
-  private KeyRequest currentKeyRequest;
-  private ProvisionRequest currentProvisionRequest;
+  private @Nullable KeyRequest currentKeyRequest;
+  private @Nullable ProvisionRequest currentProvisionRequest;
 
   /**
    * Instantiates a new DRM session.
@@ -129,18 +134,25 @@ import java.util.UUID;
       @Nullable List<SchemeData> schemeDatas,
       @DefaultDrmSessionManager.Mode int mode,
       @Nullable byte[] offlineLicenseKeySetId,
-      HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
       MediaDrmCallback callback,
       Looper playbackLooper,
       EventDispatcher<DefaultDrmSessionEventListener> eventDispatcher,
       int initialDrmRequestRetryCount) {
+    if (mode == DefaultDrmSessionManager.MODE_QUERY
+        || mode == DefaultDrmSessionManager.MODE_RELEASE) {
+      Assertions.checkNotNull(offlineLicenseKeySetId);
+    }
     this.uuid = uuid;
     this.provisioningManager = provisioningManager;
     this.mediaDrm = mediaDrm;
     this.mode = mode;
-    this.offlineLicenseKeySetId = offlineLicenseKeySetId;
-    this.schemeDatas =
-        offlineLicenseKeySetId == null ? Collections.unmodifiableList(schemeDatas) : null;
+    if (offlineLicenseKeySetId != null) {
+      this.offlineLicenseKeySetId = offlineLicenseKeySetId;
+      this.schemeDatas = null;
+    } else {
+      this.schemeDatas = Collections.unmodifiableList(Assertions.checkNotNull(schemeDatas));
+    }
     this.optionalKeyRequestParameters = optionalKeyRequestParameters;
     this.callback = callback;
     this.initialDrmRequestRetryCount = initialDrmRequestRetryCount;
@@ -166,9 +178,9 @@ import java.util.UUID;
     }
   }
 
-  /**
-   * @return True if the session is closed and cleaned up, false otherwise.
-   */
+  /** @return True if the session is closed and cleaned up, false otherwise. */
+  // Assigning null to various non-null variables for clean-up. Class won't be used after release.
+  @SuppressWarnings("assignment.type.incompatible")
   public boolean release() {
     if (--openCount == 0) {
       state = STATE_RELEASED;
@@ -245,33 +257,35 @@ import java.util.UUID;
   }
 
   @Override
-  public final DrmSessionException getError() {
+  public final @Nullable DrmSessionException getError() {
     return state == STATE_ERROR ? lastException : null;
   }
 
   @Override
-  public final T getMediaCrypto() {
+  public final @Nullable T getMediaCrypto() {
     return mediaCrypto;
   }
 
   @Override
-  public Map<String, String> queryKeyStatus() {
+  public @Nullable Map<String, String> queryKeyStatus() {
     return sessionId == null ? null : mediaDrm.queryKeyStatus(sessionId);
   }
 
   @Override
-  public byte[] getOfflineLicenseKeySetId() {
+  public @Nullable byte[] getOfflineLicenseKeySetId() {
     return offlineLicenseKeySetId;
   }
 
   // Internal methods.
 
   /**
-   *  Try to open a session, do provisioning if necessary.
-   *  @param allowProvisioning if provisioning is allowed, set this to false when calling from
-   *      processing provision response.
-   *  @return true on success, false otherwise.
+   * Try to open a session, do provisioning if necessary.
+   *
+   * @param allowProvisioning if provisioning is allowed, set this to false when calling from
+   *     processing provision response.
+   * @return true on success, false otherwise.
    */
+  @EnsuresNonNullIf(result = true, expression = "sessionId")
   private boolean openInternal(boolean allowProvisioning) {
     if (isOpen()) {
       // Already opened
@@ -319,19 +333,20 @@ import java.util.UUID;
     provisioningManager.onProvisionCompleted();
   }
 
+  @RequiresNonNull("sessionId")
   private void doLicense(boolean allowRetry) {
     switch (mode) {
       case DefaultDrmSessionManager.MODE_PLAYBACK:
       case DefaultDrmSessionManager.MODE_QUERY:
         if (offlineLicenseKeySetId == null) {
-          postKeyRequest(ExoMediaDrm.KEY_TYPE_STREAMING, allowRetry);
+          postKeyRequest(sessionId, ExoMediaDrm.KEY_TYPE_STREAMING, allowRetry);
         } else if (state == STATE_OPENED_WITH_KEYS || restoreKeys()) {
           long licenseDurationRemainingSec = getLicenseDurationRemainingSec();
           if (mode == DefaultDrmSessionManager.MODE_PLAYBACK
               && licenseDurationRemainingSec <= MAX_LICENSE_DURATION_TO_RENEW) {
             Log.d(TAG, "Offline license has expired or will expire soon. "
                 + "Remaining seconds: " + licenseDurationRemainingSec);
-            postKeyRequest(ExoMediaDrm.KEY_TYPE_OFFLINE, allowRetry);
+            postKeyRequest(sessionId, ExoMediaDrm.KEY_TYPE_OFFLINE, allowRetry);
           } else if (licenseDurationRemainingSec <= 0) {
             onError(new KeysExpiredException());
           } else {
@@ -342,19 +357,20 @@ import java.util.UUID;
         break;
       case DefaultDrmSessionManager.MODE_DOWNLOAD:
         if (offlineLicenseKeySetId == null) {
-          postKeyRequest(ExoMediaDrm.KEY_TYPE_OFFLINE, allowRetry);
+          postKeyRequest(sessionId, ExoMediaDrm.KEY_TYPE_OFFLINE, allowRetry);
         } else {
           // Renew
           if (restoreKeys()) {
-            postKeyRequest(ExoMediaDrm.KEY_TYPE_OFFLINE, allowRetry);
+            postKeyRequest(sessionId, ExoMediaDrm.KEY_TYPE_OFFLINE, allowRetry);
           }
         }
         break;
       case DefaultDrmSessionManager.MODE_RELEASE:
+        Assertions.checkNotNull(offlineLicenseKeySetId);
         // It's not necessary to restore the key (and open a session to do that) before releasing it
         // but this serves as a good sanity/fast-failure check.
         if (restoreKeys()) {
-          postKeyRequest(ExoMediaDrm.KEY_TYPE_RELEASE, allowRetry);
+          postKeyRequest(offlineLicenseKeySetId, ExoMediaDrm.KEY_TYPE_RELEASE, allowRetry);
         }
         break;
       default:
@@ -362,6 +378,7 @@ import java.util.UUID;
     }
   }
 
+  @RequiresNonNull({"sessionId", "offlineLicenseKeySetId"})
   private boolean restoreKeys() {
     try {
       mediaDrm.restoreKeys(sessionId, offlineLicenseKeySetId);
@@ -377,12 +394,12 @@ import java.util.UUID;
     if (!C.WIDEVINE_UUID.equals(uuid)) {
       return Long.MAX_VALUE;
     }
-    Pair<Long, Long> pair = WidevineUtil.getLicenseDurationRemainingSec(this);
+    Pair<Long, Long> pair =
+        Assertions.checkNotNull(WidevineUtil.getLicenseDurationRemainingSec(this));
     return Math.min(pair.first, pair.second);
   }
 
-  private void postKeyRequest(int type, boolean allowRetry) {
-    byte[] scope = type == ExoMediaDrm.KEY_TYPE_RELEASE ? offlineLicenseKeySetId : sessionId;
+  private void postKeyRequest(byte[] scope, int type, boolean allowRetry) {
     try {
       currentKeyRequest =
           mediaDrm.getKeyRequest(scope, schemeDatas, type, optionalKeyRequestParameters);
@@ -407,7 +424,7 @@ import java.util.UUID;
     try {
       byte[] responseData = (byte[]) response;
       if (mode == DefaultDrmSessionManager.MODE_RELEASE) {
-        mediaDrm.provideKeyResponse(offlineLicenseKeySetId, responseData);
+        mediaDrm.provideKeyResponse(Util.castNonNull(offlineLicenseKeySetId), responseData);
         eventDispatcher.dispatch(DefaultDrmSessionEventListener::onDrmKeysRestored);
       } else {
         byte[] keySetId = mediaDrm.provideKeyResponse(sessionId, responseData);
@@ -447,6 +464,8 @@ import java.util.UUID;
     }
   }
 
+  @EnsuresNonNullIf(result = true, expression = "sessionId")
+  @SuppressWarnings("contracts.conditional.postcondition.not.satisfied")
   private boolean isOpen() {
     return state == STATE_OPENED || state == STATE_OPENED_WITH_KEYS;
   }
@@ -461,8 +480,9 @@ import java.util.UUID;
     }
 
     @Override
+    @SuppressWarnings("unchecked")
     public void handleMessage(Message msg) {
-      Pair<?, ?> requestAndResponse = (Pair<?, ?>) msg.obj;
+      Pair<Object, Object> requestAndResponse = (Pair<Object, Object>) msg.obj;
       Object request = requestAndResponse.first;
       Object response = requestAndResponse.second;
       switch (msg.what) {
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java
index 0e751ca49e..03269f0caa 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java
@@ -21,7 +21,7 @@ import android.os.Handler;
 import android.os.Looper;
 import android.os.Message;
 import android.support.annotation.IntDef;
-import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.text.TextUtils;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.drm.DefaultDrmSession.ProvisioningManager;
@@ -94,7 +94,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   private final UUID uuid;
   private final ExoMediaDrm<T> mediaDrm;
   private final MediaDrmCallback callback;
-  private final HashMap<String, String> optionalKeyRequestParameters;
+  private final @Nullable HashMap<String, String> optionalKeyRequestParameters;
   private final EventDispatcher<DefaultDrmSessionEventListener> eventDispatcher;
   private final boolean multiSession;
   private final int initialDrmRequestRetryCount;
@@ -102,11 +102,11 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   private final List<DefaultDrmSession<T>> sessions;
   private final List<DefaultDrmSession<T>> provisioningSessions;
 
-  private Looper playbackLooper;
+  private @Nullable Looper playbackLooper;
   private int mode;
-  private byte[] offlineLicenseKeySetId;
+  private @Nullable byte[] offlineLicenseKeySetId;
 
-  /* package */ volatile MediaDrmHandler mediaDrmHandler;
+  /* package */ volatile @Nullable MediaDrmHandler mediaDrmHandler;
 
   /**
    * @deprecated Use {@link #newWidevineInstance(MediaDrmCallback, HashMap)} and {@link
@@ -115,9 +115,9 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   @Deprecated
   public static DefaultDrmSessionManager<FrameworkMediaCrypto> newWidevineInstance(
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
-      Handler eventHandler,
-      DefaultDrmSessionEventListener eventListener)
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable Handler eventHandler,
+      @Nullable DefaultDrmSessionEventListener eventListener)
       throws UnsupportedDrmException {
     DefaultDrmSessionManager<FrameworkMediaCrypto> drmSessionManager =
         newWidevineInstance(callback, optionalKeyRequestParameters);
@@ -136,7 +136,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    * @throws UnsupportedDrmException If the specified DRM scheme is not supported.
    */
   public static DefaultDrmSessionManager<FrameworkMediaCrypto> newWidevineInstance(
-      MediaDrmCallback callback, HashMap<String, String> optionalKeyRequestParameters)
+      MediaDrmCallback callback, @Nullable HashMap<String, String> optionalKeyRequestParameters)
       throws UnsupportedDrmException {
     return newFrameworkInstance(C.WIDEVINE_UUID, callback, optionalKeyRequestParameters);
   }
@@ -148,9 +148,9 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   @Deprecated
   public static DefaultDrmSessionManager<FrameworkMediaCrypto> newPlayReadyInstance(
       MediaDrmCallback callback,
-      String customData,
-      Handler eventHandler,
-      DefaultDrmSessionEventListener eventListener)
+      @Nullable String customData,
+      @Nullable Handler eventHandler,
+      @Nullable DefaultDrmSessionEventListener eventListener)
       throws UnsupportedDrmException {
     DefaultDrmSessionManager<FrameworkMediaCrypto> drmSessionManager =
         newPlayReadyInstance(callback, customData);
@@ -171,7 +171,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    * @throws UnsupportedDrmException If the specified DRM scheme is not supported.
    */
   public static DefaultDrmSessionManager<FrameworkMediaCrypto> newPlayReadyInstance(
-      MediaDrmCallback callback, String customData) throws UnsupportedDrmException {
+      MediaDrmCallback callback, @Nullable String customData) throws UnsupportedDrmException {
     HashMap<String, String> optionalKeyRequestParameters;
     if (!TextUtils.isEmpty(customData)) {
       optionalKeyRequestParameters = new HashMap<>();
@@ -190,9 +190,9 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   public static DefaultDrmSessionManager<FrameworkMediaCrypto> newFrameworkInstance(
       UUID uuid,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
-      Handler eventHandler,
-      DefaultDrmSessionEventListener eventListener)
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable Handler eventHandler,
+      @Nullable DefaultDrmSessionEventListener eventListener)
       throws UnsupportedDrmException {
     DefaultDrmSessionManager<FrameworkMediaCrypto> drmSessionManager =
         newFrameworkInstance(uuid, callback, optionalKeyRequestParameters);
@@ -212,7 +212,9 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    * @throws UnsupportedDrmException If the specified DRM scheme is not supported.
    */
   public static DefaultDrmSessionManager<FrameworkMediaCrypto> newFrameworkInstance(
-      UUID uuid, MediaDrmCallback callback, HashMap<String, String> optionalKeyRequestParameters)
+      UUID uuid,
+      MediaDrmCallback callback,
+      @Nullable HashMap<String, String> optionalKeyRequestParameters)
       throws UnsupportedDrmException {
     return new DefaultDrmSessionManager<>(
         uuid,
@@ -228,13 +230,14 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    *     and {@link #addListener(Handler, DefaultDrmSessionEventListener)}.
    */
   @Deprecated
+  @SuppressWarnings("method.invocation.invalid")
   public DefaultDrmSessionManager(
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
-      Handler eventHandler,
-      DefaultDrmSessionEventListener eventListener) {
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable Handler eventHandler,
+      @Nullable DefaultDrmSessionEventListener eventListener) {
     this(uuid, mediaDrm, callback, optionalKeyRequestParameters);
     if (eventHandler != null && eventListener != null) {
       addListener(eventHandler, eventListener);
@@ -252,7 +255,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters) {
+      @Nullable HashMap<String, String> optionalKeyRequestParameters) {
     this(
         uuid,
         mediaDrm,
@@ -267,13 +270,14 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    *     boolean)} and {@link #addListener(Handler, DefaultDrmSessionEventListener)}.
    */
   @Deprecated
+  @SuppressWarnings("method.invocation.invalid")
   public DefaultDrmSessionManager(
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
-      Handler eventHandler,
-      DefaultDrmSessionEventListener eventListener,
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable Handler eventHandler,
+      @Nullable DefaultDrmSessionEventListener eventListener,
       boolean multiSession) {
     this(uuid, mediaDrm, callback, optionalKeyRequestParameters, multiSession);
     if (eventHandler != null && eventListener != null) {
@@ -294,7 +298,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
       boolean multiSession) {
     this(
         uuid,
@@ -310,13 +314,14 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    *     boolean, int)} and {@link #addListener(Handler, DefaultDrmSessionEventListener)}.
    */
   @Deprecated
+  @SuppressWarnings("method.invocation.invalid")
   public DefaultDrmSessionManager(
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
-      Handler eventHandler,
-      DefaultDrmSessionEventListener eventListener,
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable Handler eventHandler,
+      @Nullable DefaultDrmSessionEventListener eventListener,
       boolean multiSession,
       int initialDrmRequestRetryCount) {
     this(
@@ -346,7 +351,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters,
+      @Nullable HashMap<String, String> optionalKeyRequestParameters,
       boolean multiSession,
       int initialDrmRequestRetryCount) {
     Assertions.checkNotNull(uuid);
@@ -443,21 +448,22 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
    * required.
    *
    * <p>{@code mode} must be one of these:
+   *
    * <ul>
-   * <li>{@link #MODE_PLAYBACK}: If {@code offlineLicenseKeySetId} is null, a streaming license is
-   *     requested otherwise the offline license is restored.
-   * <li>{@link #MODE_QUERY}: {@code offlineLicenseKeySetId} can not be null. The offline license
-   *     is restored.
-   * <li>{@link #MODE_DOWNLOAD}: If {@code offlineLicenseKeySetId} is null, an offline license is
-   *     requested otherwise the offline license is renewed.
-   * <li>{@link #MODE_RELEASE}: {@code offlineLicenseKeySetId} can not be null. The offline license
-   *     is released.
+   *   <li>{@link #MODE_PLAYBACK}: If {@code offlineLicenseKeySetId} is null, a streaming license is
+   *       requested otherwise the offline license is restored.
+   *   <li>{@link #MODE_QUERY}: {@code offlineLicenseKeySetId} can not be null. The offline license
+   *       is restored.
+   *   <li>{@link #MODE_DOWNLOAD}: If {@code offlineLicenseKeySetId} is null, an offline license is
+   *       requested otherwise the offline license is renewed.
+   *   <li>{@link #MODE_RELEASE}: {@code offlineLicenseKeySetId} can not be null. The offline
+   *       license is released.
    * </ul>
    *
    * @param mode The mode to be set.
    * @param offlineLicenseKeySetId The key set id of the license to be used with the given mode.
    */
-  public void setMode(@Mode int mode, byte[] offlineLicenseKeySetId) {
+  public void setMode(@Mode int mode, @Nullable byte[] offlineLicenseKeySetId) {
     Assertions.checkState(sessions.isEmpty());
     if (mode == MODE_QUERY || mode == MODE_RELEASE) {
       Assertions.checkNotNull(offlineLicenseKeySetId);
@@ -469,7 +475,7 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   // DrmSessionManager implementation.
 
   @Override
-  public boolean canAcquireSession(@NonNull DrmInitData drmInitData) {
+  public boolean canAcquireSession(DrmInitData drmInitData) {
     if (offlineLicenseKeySetId != null) {
       // An offline license can be restored so a session can always be acquired.
       return true;
@@ -650,10 +656,14 @@ public class DefaultDrmSessionManager<T extends ExoMediaCrypto> implements DrmSe
   private class MediaDrmEventListener implements OnEventListener<T> {
 
     @Override
-    public void onEvent(ExoMediaDrm<? extends T> md, byte[] sessionId, int event, int extra,
-        byte[] data) {
+    public void onEvent(
+        ExoMediaDrm<? extends T> md,
+        byte[] sessionId,
+        int event,
+        int extra,
+        @Nullable byte[] data) {
       if (mode == DefaultDrmSessionManager.MODE_PLAYBACK) {
-        mediaDrmHandler.obtainMessage(event, sessionId).sendToTarget();
+        Assertions.checkNotNull(mediaDrmHandler).obtainMessage(event, sessionId).sendToTarget();
       }
     }
 
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmInitData.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmInitData.java
index b9415c74af..ef96c7ae75 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmInitData.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmInitData.java
@@ -85,10 +85,8 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
   // Lazily initialized hashcode.
   private int hashCode;
 
-  /**
-   * The protection scheme type, or null if not applicable or unknown.
-   */
-  @Nullable public final String schemeType;
+  /** The protection scheme type, or null if not applicable or unknown. */
+  public final @Nullable String schemeType;
 
   /**
    * Number of {@link SchemeData}s.
@@ -106,7 +104,7 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
    * @param schemeType See {@link #schemeType}.
    * @param schemeDatas Scheme initialization data for possibly multiple DRM schemes.
    */
-  public DrmInitData(String schemeType, List<SchemeData> schemeDatas) {
+  public DrmInitData(@Nullable String schemeType, List<SchemeData> schemeDatas) {
     this(schemeType, false, schemeDatas.toArray(new SchemeData[schemeDatas.size()]));
   }
 
@@ -131,11 +129,11 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
     if (cloneSchemeDatas) {
       schemeDatas = schemeDatas.clone();
     }
-    // Sorting ensures that universal scheme data (i.e. data that applies to all schemes) is matched
-    // last. It's also required by the equals and hashcode implementations.
-    Arrays.sort(schemeDatas, this);
     this.schemeDatas = schemeDatas;
     schemeDataCount = schemeDatas.length;
+    // Sorting ensures that universal scheme data (i.e. data that applies to all schemes) is matched
+    // last. It's also required by the equals and hashcode implementations.
+    Arrays.sort(this.schemeDatas, this);
   }
 
   /* package */ DrmInitData(Parcel in) {
@@ -152,7 +150,7 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
    * @return The initialization data for the scheme, or null if the scheme is not supported.
    */
   @Deprecated
-  public SchemeData get(UUID uuid) {
+  public @Nullable SchemeData get(UUID uuid) {
     for (SchemeData schemeData : schemeDatas) {
       if (schemeData.matches(uuid)) {
         return schemeData;
@@ -270,10 +268,8 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
     public final @Nullable String licenseServerUrl;
     /** The mimeType of {@link #data}. */
     public final String mimeType;
-    /**
-     * The initialization data. May be null for scheme support checks only.
-     */
-    public final byte[] data;
+    /** The initialization data. May be null for scheme support checks only. */
+    public final @Nullable byte[] data;
     /**
      * Whether secure decryption is required.
      */
@@ -285,7 +281,7 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
      * @param mimeType See {@link #mimeType}.
      * @param data See {@link #data}.
      */
-    public SchemeData(UUID uuid, String mimeType, byte[] data) {
+    public SchemeData(UUID uuid, String mimeType, @Nullable byte[] data) {
       this(uuid, mimeType, data, false);
     }
 
@@ -296,7 +292,8 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
      * @param data See {@link #data}.
      * @param requiresSecureDecryption See {@link #requiresSecureDecryption}.
      */
-    public SchemeData(UUID uuid, String mimeType, byte[] data, boolean requiresSecureDecryption) {
+    public SchemeData(
+        UUID uuid, String mimeType, @Nullable byte[] data, boolean requiresSecureDecryption) {
       this(uuid, /* licenseServerUrl= */ null, mimeType, data, requiresSecureDecryption);
     }
 
@@ -312,7 +309,7 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
         UUID uuid,
         @Nullable String licenseServerUrl,
         String mimeType,
-        byte[] data,
+        @Nullable byte[] data,
         boolean requiresSecureDecryption) {
       this.uuid = Assertions.checkNotNull(uuid);
       this.licenseServerUrl = licenseServerUrl;
@@ -324,7 +321,7 @@ public final class DrmInitData implements Comparator<SchemeData>, Parcelable {
     /* package */ SchemeData(Parcel in) {
       uuid = new UUID(in.readLong(), in.readLong());
       licenseServerUrl = in.readString();
-      mimeType = in.readString();
+      mimeType = Util.castNonNull(in.readString());
       data = in.createByteArray();
       requiresSecureDecryption = in.readByte() != 0;
     }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmSession.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmSession.java
index bed3545d78..698f6fdb25 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmSession.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/DrmSession.java
@@ -18,6 +18,7 @@ package com.google.android.exoplayer2.drm;
 import android.annotation.TargetApi;
 import android.media.MediaDrm;
 import android.support.annotation.IntDef;
+import android.support.annotation.Nullable;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.util.Map;
@@ -75,21 +76,24 @@ public interface DrmSession<T extends ExoMediaCrypto> {
   @State int getState();
 
   /**
-   * Returns the cause of the error state.
+   * Returns the cause of the error state, or null if {@link #getState()} is not {@link
+   * #STATE_ERROR}.
    */
+  @Nullable
   DrmSessionException getError();
 
   /**
    * Returns a {@link ExoMediaCrypto} for the open session, or null if called before the session has
    * been opened or after it's been released.
    */
+  @Nullable
   T getMediaCrypto();
 
   /**
    * Returns a map describing the key status for the session, or null if called before the session
    * has been opened or after it's been released.
-   * <p>
-   * Since DRM license policies vary by vendor, the specific status field names are determined by
+   *
+   * <p>Since DRM license policies vary by vendor, the specific status field names are determined by
    * each DRM vendor. Refer to your DRM provider documentation for definitions of the field names
    * for a particular DRM engine plugin.
    *
@@ -97,12 +101,13 @@ public interface DrmSession<T extends ExoMediaCrypto> {
    *     has been opened or after it's been released.
    * @see MediaDrm#queryKeyStatus(byte[])
    */
+  @Nullable
   Map<String, String> queryKeyStatus();
 
   /**
    * Returns the key set id of the offline license loaded into this session, or null if there isn't
    * one.
    */
+  @Nullable
   byte[] getOfflineLicenseKeySetId();
-
 }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/ErrorStateDrmSession.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/ErrorStateDrmSession.java
index d30e670c3f..f0335b50c9 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/ErrorStateDrmSession.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/ErrorStateDrmSession.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.drm;
 
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.util.Assertions;
 import java.util.Map;
 
@@ -33,22 +34,22 @@ public final class ErrorStateDrmSession<T extends ExoMediaCrypto> implements Drm
   }
 
   @Override
-  public DrmSessionException getError() {
+  public @Nullable DrmSessionException getError() {
     return error;
   }
 
   @Override
-  public T getMediaCrypto() {
+  public @Nullable T getMediaCrypto() {
     return null;
   }
 
   @Override
-  public Map<String, String> queryKeyStatus() {
+  public @Nullable Map<String, String> queryKeyStatus() {
     return null;
   }
 
   @Override
-  public byte[] getOfflineLicenseKeySetId() {
+  public @Nullable byte[] getOfflineLicenseKeySetId() {
     return null;
   }
 
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/FrameworkMediaDrm.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/FrameworkMediaDrm.java
index 254b524b30..1f862546ae 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/FrameworkMediaDrm.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/FrameworkMediaDrm.java
@@ -131,7 +131,7 @@ public final class FrameworkMediaDrm implements ExoMediaDrm<FrameworkMediaCrypto
     String mimeType = null;
     if (schemeDatas != null) {
       schemeData = getSchemeData(uuid, schemeDatas);
-      initData = adjustRequestInitData(uuid, schemeData.data);
+      initData = adjustRequestInitData(uuid, Assertions.checkNotNull(schemeData.data));
       mimeType = adjustRequestMimeType(uuid, schemeData.mimeType);
     }
     MediaDrm.KeyRequest request =
@@ -229,11 +229,12 @@ public final class FrameworkMediaDrm implements ExoMediaDrm<FrameworkMediaCrypto
       boolean canConcatenateData = true;
       for (int i = 0; i < schemeDatas.size(); i++) {
         SchemeData schemeData = schemeDatas.get(i);
+        byte[] schemeDataData = Util.castNonNull(schemeData.data);
         if (schemeData.requiresSecureDecryption == firstSchemeData.requiresSecureDecryption
             && Util.areEqual(schemeData.mimeType, firstSchemeData.mimeType)
             && Util.areEqual(schemeData.licenseServerUrl, firstSchemeData.licenseServerUrl)
-            && PsshAtomUtil.isPsshAtom(schemeData.data)) {
-          concatenatedDataLength += schemeData.data.length;
+            && PsshAtomUtil.isPsshAtom(schemeDataData)) {
+          concatenatedDataLength += schemeDataData.length;
         } else {
           canConcatenateData = false;
           break;
@@ -244,9 +245,10 @@ public final class FrameworkMediaDrm implements ExoMediaDrm<FrameworkMediaCrypto
         int concatenatedDataPosition = 0;
         for (int i = 0; i < schemeDatas.size(); i++) {
           SchemeData schemeData = schemeDatas.get(i);
-          int schemeDataLength = schemeData.data.length;
+          byte[] schemeDataData = Util.castNonNull(schemeData.data);
+          int schemeDataLength = schemeDataData.length;
           System.arraycopy(
-              schemeData.data, 0, concatenatedData, concatenatedDataPosition, schemeDataLength);
+              schemeDataData, 0, concatenatedData, concatenatedDataPosition, schemeDataLength);
           concatenatedDataPosition += schemeDataLength;
         }
         return firstSchemeData.copyWithData(concatenatedData);
@@ -257,7 +259,7 @@ public final class FrameworkMediaDrm implements ExoMediaDrm<FrameworkMediaCrypto
     // the first V0 box.
     for (int i = 0; i < schemeDatas.size(); i++) {
       SchemeData schemeData = schemeDatas.get(i);
-      int version = PsshAtomUtil.parseVersion(schemeData.data);
+      int version = PsshAtomUtil.parseVersion(Util.castNonNull(schemeData.data));
       if (Util.SDK_INT < 23 && version == 0) {
         return schemeData;
       } else if (Util.SDK_INT >= 23 && version == 1) {
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/HttpMediaDrmCallback.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/HttpMediaDrmCallback.java
index 8b8c3bec99..4ff0af3c0e 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/HttpMediaDrmCallback.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/HttpMediaDrmCallback.java
@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.drm;
 
 import android.annotation.TargetApi;
 import android.net.Uri;
+import android.support.annotation.Nullable;
 import android.text.TextUtils;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.drm.ExoMediaDrm.KeyRequest;
@@ -135,8 +136,12 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
     return executePost(dataSourceFactory, url, request.getData(), requestProperties);
   }
 
-  private static byte[] executePost(HttpDataSource.Factory dataSourceFactory, String url,
-      byte[] data, Map<String, String> requestProperties) throws IOException {
+  private static byte[] executePost(
+      HttpDataSource.Factory dataSourceFactory,
+      String url,
+      byte[] data,
+      @Nullable Map<String, String> requestProperties)
+      throws IOException {
     HttpDataSource dataSource = dataSourceFactory.createDataSource();
     if (requestProperties != null) {
       for (Map.Entry<String, String> requestProperty : requestProperties.entrySet()) {
@@ -164,17 +169,18 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
         boolean manuallyRedirect =
             (e.responseCode == 307 || e.responseCode == 308)
                 && manualRedirectCount++ < MAX_MANUAL_REDIRECTS;
-        url = manuallyRedirect ? getRedirectUrl(e) : null;
-        if (url == null) {
+        String redirectUrl = manuallyRedirect ? getRedirectUrl(e) : null;
+        if (redirectUrl == null) {
           throw e;
         }
+        url = redirectUrl;
       } finally {
         Util.closeQuietly(inputStream);
       }
     }
   }
 
-  private static String getRedirectUrl(InvalidResponseCodeException exception) {
+  private static @Nullable String getRedirectUrl(InvalidResponseCodeException exception) {
     Map<String, List<String>> headerFields = exception.headerFields;
     if (headerFields != null) {
       List<String> locationHeaders = headerFields.get("Location");
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/OfflineLicenseHelper.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/OfflineLicenseHelper.java
index 9298c16cb0..d596b4ab90 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/OfflineLicenseHelper.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/OfflineLicenseHelper.java
@@ -19,6 +19,7 @@ import android.media.MediaDrm;
 import android.os.ConditionVariable;
 import android.os.Handler;
 import android.os.HandlerThread;
+import android.support.annotation.Nullable;
 import android.util.Pair;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.drm.DefaultDrmSessionManager.Mode;
@@ -34,6 +35,8 @@ import java.util.UUID;
  */
 public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
 
+  private static final DrmInitData DUMMY_DRM_INIT_DATA = new DrmInitData();
+
   private final ConditionVariable conditionVariable;
   private final DefaultDrmSessionManager<T> drmSessionManager;
   private final HandlerThread handlerThread;
@@ -95,7 +98,7 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
       String defaultLicenseUrl,
       boolean forceDefaultLicenseUrl,
       Factory httpDataSourceFactory,
-      HashMap<String, String> optionalKeyRequestParameters)
+      @Nullable HashMap<String, String> optionalKeyRequestParameters)
       throws UnsupportedDrmException {
     return new OfflineLicenseHelper<>(C.WIDEVINE_UUID,
         FrameworkMediaDrm.newInstance(C.WIDEVINE_UUID),
@@ -118,7 +121,7 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
       UUID uuid,
       ExoMediaDrm<T> mediaDrm,
       MediaDrmCallback callback,
-      HashMap<String, String> optionalKeyRequestParameters) {
+      @Nullable HashMap<String, String> optionalKeyRequestParameters) {
     handlerThread = new HandlerThread("OfflineLicenseHelper");
     handlerThread.start();
     conditionVariable = new ConditionVariable();
@@ -199,7 +202,8 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
   public synchronized byte[] renewLicense(byte[] offlineLicenseKeySetId)
       throws DrmSessionException {
     Assertions.checkNotNull(offlineLicenseKeySetId);
-    return blockingKeyRequest(DefaultDrmSessionManager.MODE_DOWNLOAD, offlineLicenseKeySetId, null);
+    return blockingKeyRequest(
+        DefaultDrmSessionManager.MODE_DOWNLOAD, offlineLicenseKeySetId, DUMMY_DRM_INIT_DATA);
   }
 
   /**
@@ -211,7 +215,8 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
   public synchronized void releaseLicense(byte[] offlineLicenseKeySetId)
       throws DrmSessionException {
     Assertions.checkNotNull(offlineLicenseKeySetId);
-    blockingKeyRequest(DefaultDrmSessionManager.MODE_RELEASE, offlineLicenseKeySetId, null);
+    blockingKeyRequest(
+        DefaultDrmSessionManager.MODE_RELEASE, offlineLicenseKeySetId, DUMMY_DRM_INIT_DATA);
   }
 
   /**
@@ -224,8 +229,9 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
   public synchronized Pair<Long, Long> getLicenseDurationRemainingSec(byte[] offlineLicenseKeySetId)
       throws DrmSessionException {
     Assertions.checkNotNull(offlineLicenseKeySetId);
-    DrmSession<T> drmSession = openBlockingKeyRequest(DefaultDrmSessionManager.MODE_QUERY,
-        offlineLicenseKeySetId, null);
+    DrmSession<T> drmSession =
+        openBlockingKeyRequest(
+            DefaultDrmSessionManager.MODE_QUERY, offlineLicenseKeySetId, DUMMY_DRM_INIT_DATA);
     DrmSessionException error = drmSession.getError();
     Pair<Long, Long> licenseDurationRemainingSec =
         WidevineUtil.getLicenseDurationRemainingSec(drmSession);
@@ -236,7 +242,7 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
       }
       throw error;
     }
-    return licenseDurationRemainingSec;
+    return Assertions.checkNotNull(licenseDurationRemainingSec);
   }
 
   /**
@@ -246,8 +252,9 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
     handlerThread.quit();
   }
 
-  private byte[] blockingKeyRequest(@Mode int licenseMode, byte[] offlineLicenseKeySetId,
-      DrmInitData drmInitData) throws DrmSessionException {
+  private byte[] blockingKeyRequest(
+      @Mode int licenseMode, @Nullable byte[] offlineLicenseKeySetId, DrmInitData drmInitData)
+      throws DrmSessionException {
     DrmSession<T> drmSession = openBlockingKeyRequest(licenseMode, offlineLicenseKeySetId,
         drmInitData);
     DrmSessionException error = drmSession.getError();
@@ -256,11 +263,11 @@ public final class OfflineLicenseHelper<T extends ExoMediaCrypto> {
     if (error != null) {
       throw error;
     }
-    return keySetId;
+    return Assertions.checkNotNull(keySetId);
   }
 
-  private DrmSession<T> openBlockingKeyRequest(@Mode int licenseMode, byte[] offlineLicenseKeySetId,
-      DrmInitData drmInitData) {
+  private DrmSession<T> openBlockingKeyRequest(
+      @Mode int licenseMode, @Nullable byte[] offlineLicenseKeySetId, DrmInitData drmInitData) {
     drmSessionManager.setMode(licenseMode, offlineLicenseKeySetId);
     conditionVariable.close();
     DrmSession<T> drmSession = drmSessionManager.acquireSession(handlerThread.getLooper(),
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/drm/WidevineUtil.java b/library/core/src/main/java/com/google/android/exoplayer2/drm/WidevineUtil.java
index 45c38b3609..b8b80490a1 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/drm/WidevineUtil.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/drm/WidevineUtil.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.drm;
 
+import android.support.annotation.Nullable;
 import android.util.Pair;
 import com.google.android.exoplayer2.C;
 import java.util.Map;
@@ -38,7 +39,8 @@ public final class WidevineUtil {
    * @return A {@link Pair} consisting of the remaining license and playback durations in seconds,
    *     or null if called before the session has been opened or after it's been released.
    */
-  public static Pair<Long, Long> getLicenseDurationRemainingSec(DrmSession<?> drmSession) {
+  public static @Nullable Pair<Long, Long> getLicenseDurationRemainingSec(
+      DrmSession<?> drmSession) {
     Map<String, String> keyStatus = drmSession.queryKeyStatus();
     if (keyStatus == null) {
       return null;
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/drm/OfflineLicenseHelperTest.java b/library/core/src/test/java/com/google/android/exoplayer2/drm/OfflineLicenseHelperTest.java
index 6f62b7fcfc..bba487f321 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/drm/OfflineLicenseHelperTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/drm/OfflineLicenseHelperTest.java
@@ -92,9 +92,12 @@ public class OfflineLicenseHelperTest {
   public void testDownloadLicenseFailsIfNoKeySetIdIsReturned() throws Exception {
     setStubLicenseAndPlaybackDurationValues(1000, 200);
 
-    byte[] offlineLicenseKeySetId = offlineLicenseHelper.downloadLicense(newDrmInitData());
-
-    assertThat(offlineLicenseKeySetId).isNull();
+    try {
+      offlineLicenseHelper.downloadLicense(newDrmInitData());
+      fail();
+    } catch (Exception e) {
+      // Expected.
+    }
   }
 
   @Test