final

pp.A4-HandOverHandLocking/src/main/java/pp/Main.java

@@ -18,25 +18,25 @@ public class Main {
     }
 
     public static void main(String... args) throws InterruptedException {
-        list = new ThreadsafeSimplifiedList<>();
-        for (int i = 0; i < 5000; i++) {
-            list.add(i);
-        }
-        var thread0 = new Thread(sliceSum(0, 1250, 780625));
-        var thread1 = new Thread(sliceSum(1250, 2500, 2343125));
-        var thread2 = new Thread(sliceSum(2500, 3750, 3905625));
-        var thread3 = new Thread(sliceSum(3750, 5000, 5468125));
-        var start = System.currentTimeMillis();
-        thread0.start();
-        thread1.start();
-        thread2.start();
-        thread3.start();
-        thread0.join();
-        thread1.join();
-        thread2.join();
-        thread3.join();
-        System.out.printf("%s: %d ms\n", list.getClass().toString(),
-                System.currentTimeMillis() - start);
+    	  list = new ThreadsafeSimplifiedList<>();
+          for (int i = 0; i < 5000; i++) {
+              list.add(i);
+          }
+          var thread0 = new Thread(sliceSum(0, 1250, 780625));
+          var thread1 = new Thread(sliceSum(1250, 2500, 2343125));
+          var thread2 = new Thread(sliceSum(2500, 3750, 3905625));
+          var thread3 = new Thread(sliceSum(3750, 5000, 5468125));
+          var start = System.currentTimeMillis();
+          thread0.start();
+          thread1.start();
+          thread2.start();
+          thread3.start();
+          thread0.join();
+          thread1.join();
+          thread2.join();
+          thread3.join();
+          System.out.printf("%s: %d ms\n", list.getClass().toString(),
+                  System.currentTimeMillis() - start);
     }
 
 }

pp.A4-HandOverHandLocking/src/main/java/pp/ThreadsafeSimplifiedList.java

@@ -1,97 +1,144 @@
 package pp;
 
-public class ThreadsafeSimplifiedList<T> implements SimplifiedList<T> {
-  private Node<T> first;
-  private final Object headLock = new Object();
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
-  private static class Node<U> {
-    U element;
-    Node<U> prev;
-    Node<U> next;
-    final Object lock = new Object();
+public class ThreadsafeSimplifiedList<T>
+        implements SimplifiedList<T> {
 
-    Node(U element, Node<U> prev, Node<U> next) {
+  private Node<T> head;
+  private final Lock listLock = new ReentrantLock();
+
+  private class Node<U> {
+    private U element;
+    private Node<U> next;
+    private final Lock nodeLock;
+
+    private Node(U element, Node<U> prev, Node<U> next) {
+      super();
       this.element = element;
-      this.prev = prev;
       this.next = next;
+      this.nodeLock = new ReentrantLock();
     }
   }
 
-  public ThreadsafeSimplifiedList() {
-    this.first = null;
-  }
-
-  @Override
-  public boolean add(T element) {
-    Node<T> curr;
-
-    synchronized (headLock) {
-      if (first == null) {
-        first = new Node<>(element, null, null);
-        return true;
-      }
-      curr = first;
-    }
-
-    while (true) {
-      Node<T> next;
-      synchronized (curr.lock) {
-        next = curr.next;
-        if (next == null) {
-          curr.next = new Node<>(element, curr, null);
-          return true;
-        }
-      }
-      curr = next;
-    }
-  }
-
+  /**
+   * Returns the element at the specified position in this list.
+   *
+   * @param index index of the element to return
+   * @return the element at the specified position in this list
+   */
   @Override
   public T get(int index) {
-    Node<T> curr;
-    synchronized (headLock) {
-      if (first == null) throw new IndexOutOfBoundsException();
-      curr = first;
+    this.listLock.lock();
+    var ptr = this.head;
+    if (ptr == null) {
+      this.listLock.unlock();
+      throw new IndexOutOfBoundsException(index + " out of bounds (empty list)");
+    }
+    ptr.nodeLock.lock();
+    this.listLock.unlock();
+    for (var i = 0; i < index; i++) {
+      if (ptr.next != null) {
+        ptr.next.nodeLock.lock();
+        var savePtr = ptr;
+        ptr = ptr.next;
+        savePtr.nodeLock.unlock();
+      } else {
+        ptr.nodeLock.unlock();
+        throw new IndexOutOfBoundsException(index + " out of bounds");
+      }
     }
 
-    synchronized (curr.lock) {
-      for (int i = 0; i < index; i++) {
-        Node<T> next = curr.next;
-        if (next == null) throw new IndexOutOfBoundsException();
-        synchronized (next.lock) {
-          curr = next;
-        }
-      }
-      return delay(curr.element);
+    try {
+      return delay(ptr.element);
+    } finally {
+      ptr.nodeLock.unlock();
     }
   }
 
+  /**
+   * Appends the specified element to the end of this list. There are no
+   * limitations on what elements may be added to this list.
+   *
+   * @param element element to be appended to this list
+   * @return true
+   * @see java.util.Collection#add(Object)
+   *
+   */
+  @Override
+  public boolean add(T element) {
+    this.listLock.lock();
+    if (this.head != null) {
+      var ptr = this.head;
+      ptr.nodeLock.lock();
+      this.listLock.unlock();
+      while (ptr.next != null) {
+        ptr.next.nodeLock.lock();
+        var savePtr = ptr;
+        ptr = ptr.next;
+        savePtr.nodeLock.unlock();
+      }
+      ptr.next = new Node<>(element, ptr, null);
+      ptr.nodeLock.unlock();
+    } else {
+      this.head = new Node<>(element, null, null);
+      this.listLock.unlock();
+    }
+    return true;
+  }
+
+  /**
+   * Replaces the element at the specified position in this list with the
+   * specified element.
+   *
+   * @param index   index of the element to replace
+   * @param element element to be stored at the specified position
+   * @return the element previously at the specified position
+   */
   @Override
   public T set(int index, T element) {
-    Node<T> curr;
-    synchronized (headLock) {
-      if (first == null) throw new IndexOutOfBoundsException();
-      curr = first;
+    this.listLock.lock();
+    var ptr = this.head;
+    if (ptr == null) {
+      this.listLock.unlock();
+      throw new IndexOutOfBoundsException(index + " out of bounds (empty list)");
+    }
+    ptr.nodeLock.lock();
+    this.listLock.unlock();
+    for (var i = 0; i < index; i++) {
+      if (ptr.next != null) {
+        ptr.next.nodeLock.lock();
+        var savePtr = ptr;
+        ptr = ptr.next;
+        savePtr.nodeLock.unlock();
+      } else {
+        ptr.nodeLock.unlock();
+        throw new IndexOutOfBoundsException(index + " out of bounds");
+      }
     }
 
-    synchronized (curr.lock) {
-      for (int i = 0; i < index; i++) {
-        Node<T> next = curr.next;
-        if (next == null) throw new IndexOutOfBoundsException();
-        synchronized (next.lock) {
-          curr = next;
-        }
-      }
-      T old = curr.element;
-      curr.element = element;
+    try {
+      T old = ptr.element;
+      ptr.element = element;
       return old;
+    } finally {
+      ptr.nodeLock.unlock();
     }
   }
 
+  /**
+   * Returns true if this list contains no elements.
+   *
+   * @return true if this list contains no elements
+   */
   @Override
   public boolean isEmpty() {
-    synchronized (headLock) {
-      return first == null;
+    this.listLock.lock();
+    try {
+      return this.head == null;
+    } finally {
+      this.listLock.unlock();
     }
   }
 }