1_基础知识_chapter03_对象的共享_3_线程封闭

• (1) 访问共享的可变数据时, 通常就需要同步

  (2) 为了避免同步, 可以不共享数据, 在单线程内访问拘束的话就不需要共享数据, 这就叫线程封闭

• 线程封闭的方式

  (1) Ad-hoc封闭: 完全交给程序实现者承担

  尽量少用这种方式

  (2) 栈封闭: 只能通过局部变量访问对象

  示例

    public class Animals {
  
        ...
  
        public int loadTheArk(Collection candidates) {
  
            SortedSet animals;
            int numPairs = 0;
            Animal candidate = null;
  
            // animals confined to method, don't let them escape!
            animals = new TreeSet(new SpeciesGenderComparator());
            animals.addAll(candidates);
    
            for (Animal a : animals) {
  
                if (candidate == null || !candidate.isPotentialMate(a)) {
                    candidate = a;
                } else {
                    ark.load(new AnimalPair(candidate, a));
                    ++numPairs;
                    candidate = null;
                }
            }
            
            return numPairs;
        }
    }
  

  这个示例中, 输入参数 candidates 是共享的数据, 而栈封闭的意思是访问对象只能通过局部变量, 因此示例中使用局部变量animals作为candidates的副本，所有的访问都发生在局部变量animals上, 这样根本就不会共享数据。

  使用栈封闭的风险是: 需求必须明确, 否则很有可能被后续的维护人员错误将对象逸出(例如示例中将candidates或者animals传到了其他线程中)

  (2) 使用ThreadLocal类

  1° JavaDoc说明

  This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its {@code get} or {@code set} method) has its own, independently initialized copy of the variable. {@code ThreadLocal} instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).

  For example, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokes {@code ThreadId.get()} and remains unchanged on subsequent calls.

    class ThreadId {
  
        // Atomic integer containing the next thread ID to be assigned
        private static final AtomicInteger nextId = new AtomicInteger(0);
  
        // Thread local variable containing each thread's ID
        private static final ThreadLocal threadId = new ThreadLocal() {
                @Override
                protected Integer initialValue() {
                    return nextId.getAndIncrement();
                }
            };
  
        // Returns the current thread's unique ID, assigning it if necessary
        public static int get() {
  
            return threadId.get();
        }
    }
  

  Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the {@code ThreadLocal} instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).

  2° 从概念上看, ThreadLocal相当于内置了Map, 其中保存了特定于当前线程的值

  3° 如果需要将一个单线程应用程序移植到多线程环境中，可以将共享的变量转换为ThreadLocal对象

  4° 示例

    public class ConnectionDispenser {
  
        static String DB_URL = "jdbc:mysql://localhost/mydatabase";
  
        private ThreadLocal connectionHolder = new ThreadLocal() {
  
            @Override
            protected Connection initialValue() {
  
                try {
                    return DriverManager.getConnection(DB_URL);
                } catch (SQLException e) {
                    throw new RuntimeException("Unable to acquire Connection, e");
                }
            }
        };
  
  
        public Connection getConnection() {
  
            return connectionHolder.get();
        }
    }
  

  这样, 每个线程都会拥有属于自己的Connection对象副本, 它们互不干扰