Java线程安全单例类

Question

我写了下面的Singleton类。我不确定这是否是线程安全的单例类吗？

public class CassandraAstyanaxConnection {

    private static CassandraAstyanaxConnection _instance;
    private AstyanaxContext<Keyspace> context;
    private Keyspace keyspace;
    private ColumnFamily<String, String> emp_cf;



    public static synchronized CassandraAstyanaxConnection getInstance() {
        if (_instance == null) {
            _instance = new CassandraAstyanaxConnection();
        }
        return _instance;
    }

    /**
     * Creating Cassandra connection using Astyanax client
     *
     */
    private CassandraAstyanaxConnection() {

        context = new AstyanaxContext.Builder()
        .forCluster(ModelConstants.CLUSTER)
        .forKeyspace(ModelConstants.KEYSPACE)
        .withAstyanaxConfiguration(new AstyanaxConfigurationImpl()      
            .setDiscoveryType(NodeDiscoveryType.RING_DESCRIBE)
        )
        .withConnectionPoolConfiguration(new ConnectionPoolConfigurationImpl("MyConnectionPool")
            .setPort(9160)
            .setMaxConnsPerHost(1)
            .setSeeds("127.0.0.1:9160")
        )
        .withAstyanaxConfiguration(new AstyanaxConfigurationImpl()      
            .setCqlVersion("3.0.0")
            .setTargetCassandraVersion("1.2"))
        .withConnectionPoolMonitor(new CountingConnectionPoolMonitor())
        .buildKeyspace(ThriftFamilyFactory.getInstance());

        context.start();
        keyspace = context.getEntity();

        emp_cf = ColumnFamily.newColumnFamily(
            ModelConstants.COLUMN_FAMILY, 
            StringSerializer.get(), 
            StringSerializer.get());
    }

    /**
     * returns the keyspace
     * 
     * @return
     */
    public Keyspace getKeyspace() {
        return keyspace;
    }

    public ColumnFamily<String, String> getEmp_cf() {
        return emp_cf;
    }
}

谁能帮我这个？我在上述Singleton课堂上的任何想法都会有很大帮助。

更新代码：

我正在尝试将波希米亚建议纳入我的代码中。这是更新的代码，我得到了-

public class CassandraAstyanaxConnection {
    private static class ConnectionHolder {
        static final CassandraAstyanaxConnection connection = new CassandraAstyanaxConnection();
    }
    public static CassandraAstyanaxConnection getInstance() {
        return ConnectionHolder.connection;
    }
    /**
     * Creating Cassandra connection using Astyanax client
     *
     */
    private CassandraAstyanaxConnection() {
        context = new AstyanaxContext.Builder()
        .forCluster(ModelConstants.CLUSTER)
        .forKeyspace(ModelConstants.KEYSPACE)
        .withAstyanaxConfiguration(new AstyanaxConfigurationImpl()      
        .setDiscoveryType(NodeDiscoveryType.RING_DESCRIBE)
                )
                .withConnectionPoolConfiguration(new ConnectionPoolConfigurationImpl("MyConnectionPool")
                .setPort(9160)
                .setMaxConnsPerHost(1)
                .setSeeds("127.0.0.1:9160")
                        )
                        .withAstyanaxConfiguration(new AstyanaxConfigurationImpl()      
                        .setCqlVersion("3.0.0")
                        .setTargetCassandraVersion("1.2"))
                        .withConnectionPoolMonitor(new CountingConnectionPoolMonitor())
                        .buildKeyspace(ThriftFamilyFactory.getInstance());
        context.start();
        keyspace = context.getEntity();
        emp_cf = ColumnFamily.newColumnFamily(
                ModelConstants.COLUMN_FAMILY, 
                StringSerializer.get(), 
                StringSerializer.get());
    }
    /**
     * returns the keyspace
     * 
     * @return
     */
    public Keyspace getKeyspace() {
        return keyspace;
    }
    public ColumnFamily<String, String> getEmp_cf() {
        return emp_cf;
    }
}

谁能看一下，让我知道这次是否正确？

Answer 1

但是有一个简单的技巧，可以让你的代码是线程安全的实现，并不需要同步！它称为“ 按需初始化持有人”习惯用法，它看起来像这样：

public class CassandraAstyanaxConnection {

    private CassandraAstyanaxConnection(){ }        

    private static class Holder {
       private static final CassandraAstyanaxConnection INSTANCE = new CassandraAstyanaxConnection();
    }

    public static CassandraAstyanaxConnection getInstance() {
        return Holder.INSTANCE;
    }
    // rest of class omitted
}

此代码在首次调用时初始化实例getInstance()，并且重要的是由于类加载器的约定，不需要同步：

• 类加载器在首次访问类时加载类（在这种情况下Holder，唯一的访问是在getInstance()方法内）
• 当加载了一个类，并且在任何人都可以使用它之前，请确保所有静态初始化程序都将被执行（这是在Holder静态块触发时）
• 类加载器具有自己内置的同步功能，可以确保上述两点是线程安全的
  每当需要延迟初始化时，我都会使用它来做一个巧妙的小技巧。final即使实例是延迟创建的，你也可以获得实例的奖励。还要注意代码是多么干净和简单。
  

编辑：你应该将所有构造函数设置为私有或受保护。设置并清空私有构造函数即可完成工作