Feedback.

launcher/src/main/java/org/apache/spark/launcher/ChildProcAppHandle.java

@@ -94,7 +94,6 @@ void monitorChild() {
         return;
       }
 
-      State currState = getState();
       disconnect();
 
       int ec;
@@ -105,6 +104,7 @@ void monitorChild() {
         ec = 1;
       }
 
+      State currState = getState();
       State newState = null;
       if (ec != 0) {
         // Override state with failure if the current state is not final, or is success.

launcher/src/main/java/org/apache/spark/launcher/InProcessAppHandle.java

@@ -19,14 +19,18 @@
 
 import java.io.IOException;
 import java.lang.reflect.Method;
-import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
 class InProcessAppHandle extends AbstractAppHandle {
 
+  private static final String THREAD_NAME_FMT = "spark-app-%d: '%s'";
   private static final Logger LOG = Logger.getLogger(ChildProcAppHandle.class.getName());
-  private static final ThreadFactory THREAD_FACTORY = new NamedThreadFactory("spark-app-%d");
+  private static final AtomicLong THREAD_IDS = new AtomicLong();
+
+  // Avoid really long thread names.
+  private static final int MAX_APP_NAME_LEN = 16;
 
   private Thread app;
 
@@ -47,9 +51,14 @@ public synchronized void kill() {
     setState(State.KILLED);
   }
 
-  synchronized void start(Method main, String[] args) {
+  synchronized void start(String appName, Method main, String[] args) {
     CommandBuilderUtils.checkState(app == null, "Handle already started.");
-    app = THREAD_FACTORY.newThread(() -> {
+
+    if (appName.length() > MAX_APP_NAME_LEN) {
+      appName = "..." + appName.substring(appName.length() - MAX_APP_NAME_LEN);
+    }
+
+    app = new Thread(() -> {
       try {
         main.invoke(null, (Object) args);
       } catch (Throwable t) {
@@ -59,15 +68,15 @@ synchronized void start(Method main, String[] args) {
 
       synchronized (InProcessAppHandle.this) {
         if (!isDisposed()) {
-          State currState = getState();
           disconnect();
-
-          if (!currState.isFinal()) {
+          if (!getState().isFinal()) {
             setState(State.LOST, true);
           }
         }
       }
     });
+
+    app.setName(String.format(THREAD_NAME_FMT, THREAD_IDS.incrementAndGet(), appName));
     app.start();
   }
 

launcher/src/main/java/org/apache/spark/launcher/InProcessLauncher.java

@@ -70,7 +70,9 @@ public SparkAppHandle startApplication(SparkAppHandle.Listener... listeners) thr
     List<String> sparkArgs = builder.buildSparkSubmitArgs();
     String[] argv = sparkArgs.toArray(new String[sparkArgs.size()]);
 
-    handle.start(main, argv);
+    String appName = CommandBuilderUtils.firstNonEmpty(builder.appName, builder.mainClass,
+      "<unknown>");
+    handle.start(appName, main, argv);
     return handle;
   }
 

launcher/src/main/java/org/apache/spark/launcher/LauncherServer.java

@@ -107,7 +107,7 @@ static synchronized LauncherServer getServer() {
 
   private final AtomicLong refCount;
   private final AtomicLong threadIds;
-  private final ConcurrentMap<String, AbstractAppHandle> pending;
+  private final ConcurrentMap<String, AbstractAppHandle> secretToPendingApps;
   private final List<ServerConnection> clients;
   private final ServerSocket server;
   private final Thread serverThread;
@@ -127,7 +127,7 @@ private LauncherServer() throws IOException {
       this.clients = new ArrayList<>();
       this.threadIds = new AtomicLong();
       this.factory = new NamedThreadFactory(THREAD_NAME_FMT);
-      this.pending = new ConcurrentHashMap<>();
+      this.secretToPendingApps = new ConcurrentHashMap<>();
       this.timeoutTimer = new Timer("LauncherServer-TimeoutTimer", true);
       this.server = server;
       this.running = true;
@@ -149,7 +149,7 @@ private LauncherServer() throws IOException {
    */
   synchronized String registerHandle(AbstractAppHandle handle) {
     String secret = createSecret();
-    pending.put(secret, handle);
+    secretToPendingApps.put(secret, handle);
     return secret;
   }
 
@@ -210,9 +210,10 @@ int getPort() {
    * the server.
    */
   void unregister(AbstractAppHandle handle) {
-    for (Map.Entry<String, AbstractAppHandle> e : pending.entrySet()) {
+    for (Map.Entry<String, AbstractAppHandle> e : secretToPendingApps.entrySet()) {
       if (e.getValue().equals(handle)) {
-        pending.remove(e.getKey());
+        String secret = e.getKey();
+        secretToPendingApps.remove(secret);
         break;
       }
     }
@@ -278,7 +279,7 @@ private String createSecret() {
       }
 
       String secretStr = sb.toString();
-      if (!pending.containsKey(secretStr)) {
+      if (!secretToPendingApps.containsKey(secretStr)) {
         return secretStr;
       }
     }
@@ -301,7 +302,7 @@ protected void handle(Message msg) throws IOException {
           timeout.cancel();
           timeout = null;
           Hello hello = (Hello) msg;
-          AbstractAppHandle handle = pending.remove(hello.secret);
+          AbstractAppHandle handle = secretToPendingApps.remove(hello.secret);
           if (handle != null) {
             handle.setConnection(this);
             handle.setState(SparkAppHandle.State.CONNECTED);

launcher/src/main/java/org/apache/spark/launcher/package-info.java

@@ -16,17 +16,18 @@
  */
 
 /**
- * Library for launching Spark applications.
+ * Library for launching Spark applications programmatically.
  *
  * <p>
- * This library allows applications to launch Spark programmatically. There's only one entry
- * point to the library - the {@link org.apache.spark.launcher.SparkLauncher} class.
+ * There are two ways to start applications with this library: as a child process, using
+ * {@link org.apache.spark.launcher.SparkLauncher}, or in-process, using
+ * {@link org.apache.spark.launcher.InProcessLauncher}.
  * </p>
  *
  * <p>
- * The {@link org.apache.spark.launcher.SparkLauncher#startApplication(
- * org.apache.spark.launcher.SparkAppHandle.Listener...)} can be used to start Spark and provide
- * a handle to monitor and control the running application:
+ * The {@link org.apache.spark.launcher.AbstractLauncher#startApplication(
+ * org.apache.spark.launcher.SparkAppHandle.Listener...)}  method can be used to start Spark and
+ * provide a handle to monitor and control the running application:
  * </p>
  *
  * <pre>
@@ -49,16 +50,20 @@
  * </pre>
  *
  * <p>
- * Applications can also be launched in-process by using
- * {@link org.apache.spark.launcher.InProcessLauncher} instead. Launching applications in-process
- * is only recommended in cluster mode, since Spark cannot run multiple client-mode applications
- * concurrently in the same process. The in-process launcher requires the necessary Spark
- * dependencies (such as spark-core and cluster manager-specific modules) to be present in the
- * caller thread's class loader.
+ * Launching applications as a child process requires a full Spark installation. The installation
+ * directory can be provided to the launcher explicitly in the launcher's configuration, or by
+ * setting the <i>SPARK_HOME</i> environment variable.
  * </p>
  *
  * <p>
- * It's also possible to launch a raw child process, using the
+ * Launching applications in-process is only recommended in cluster mode, since Spark cannot run
+ * multiple client-mode applications concurrently in the same process. The in-process launcher
+ * requires the necessary Spark dependencies (such as spark-core and cluster manager-specific
+ * modules) to be present in the caller thread's class loader.
+ * </p>
+ *
+ * <p>
+ * It's also possible to launch a raw child process, without the extra monitoring, using the
  * {@link org.apache.spark.launcher.SparkLauncher#launch()} method:
  * </p>
  *