第十章 避免活跃性危险

XWxiaowei · XWxiaowei · commit fd61e7e2b0be · 2017-08-28T11:06:57.000+08:00
diff --git a/java-thread-demo/src/main/com/jay/thread/chap10/DemonstrateDeadlock.java b/java-thread-demo/src/main/com/jay/thread/chap10/DemonstrateDeadlock.java
@@ -0,0 +1,69 @@
+package com.jay.thread.chap10;
+
+/**
+ * 动态的锁顺序死锁，容易发生死锁
+ * 例如：如果两个线程同时调用transferMoney,
+ * 其中一个线程从X向Y转账，另一个线程从Y向X转账，
+ * 那么就会发生死锁
+ * A：transferMoney(myAccount,yourAccount,10)
+ * B:transferMoney(yourAccount,myAccount,20)
+ * <p>
+ * 如果执行时序不当，那么A可能获得myAccount的锁并等待yourAccount
+ * 的锁，然而B此时持有yourAccount的锁，并正在等待myAccount的锁
+ * Created by xiang.wei on 2017/8/28
+ */
+public class DemonstrateDeadlock {
+
+    public static void transferMoney(Account fromAccount,
+                                     Account toAccount,
+                                     DollarAmount amount) throws InsufficentFundsException {
+        synchronized (fromAccount) {
+            synchronized (toAccount) {
+                if (fromAccount.getBalance().compareTo(amount)<0) {
+                    throw new InsufficentFundsException();
+                } else {
+                    fromAccount.debit(amount);
+                    toAccount.credit(amount);
+                }
+            }
+        }
+
+    }
+
+    static class DollarAmount implements Comparable<DollarAmount> {
+        public DollarAmount add(DollarAmount amount) {
+            return null;
+        }
+
+        public DollarAmount subtract(DollarAmount amount) {
+            return null;
+        }
+
+        @Override
+        public int compareTo(DollarAmount o) {
+            return 0;
+        }
+    }
+
+
+    static class Account {
+        private DollarAmount balance;
+
+        void debit(DollarAmount amount) {
+            balance = balance.subtract(amount);
+        }
+
+        void credit(DollarAmount amount) {
+            balance = balance.add(amount);
+        }
+
+        public DollarAmount getBalance() {
+            return balance;
+        }
+    }
+
+    static class InsufficentFundsException extends Exception {
+    }
+
+
+}
diff --git a/java-thread-demo/src/main/com/jay/thread/chap10/InduceLockOrder.java b/java-thread-demo/src/main/com/jay/thread/chap10/InduceLockOrder.java
@@ -0,0 +1,11 @@
+package com.jay.thread.chap10;
+
+/**
+ * 在制定锁的顺序时，可以使用System.identityHashCode方法，该方法将返回有Object.hashCode
+ * 返回的值。
+ * Created by xiang.wei on 2017/8/28
+ */
+public class InduceLockOrder {
+
+
+}
diff --git a/java-thread-demo/src/main/com/jay/thread/chap10/LeftRightDeadlock.java b/java-thread-demo/src/main/com/jay/thread/chap10/LeftRightDeadlock.java
@@ -0,0 +1,33 @@
+package com.jay.thread.chap10;
+
+/**
+ * 容易发生死锁！锁顺序死锁
+ * Created by xiang.wei on 2017/8/28
+ */
+public class LeftRightDeadlock {
+    private final Object left = new Object();
+    private final Object right = new Object();
+
+    public void leftRight() {
+        synchronized (left) {
+            synchronized (right) {
+                doSomething();
+            }
+        }
+    }
+
+    public void rightLeft() {
+        synchronized (right) {
+            synchronized (left) {
+                doSomethingElse();
+            }
+        }
+    }
+
+    public void doSomething() {
+    }
+
+    public void doSomethingElse() {
+
+    }
+}
diff --git a/java-thread-demo/src/main/com/jay/thread/chap10/避免活跃性危险.md b/java-thread-demo/src/main/com/jay/thread/chap10/避免活跃性危险.md
@@ -0,0 +1,8 @@
+**死锁**
+
+例如线程A在等待线程B释放其持有的资源，而线程B永远都不释放该资源
+那么线程A就会一直等待下去。典型的例子就是"哲学家进餐"
+5个哲学家，5根筷子，并且每两个人中间放一根筷子。哲学家们
+时而思考，时而进餐。
+
+
