JAVA单例问题分析

ZDNet至顶网软件频道 这个问题由最开始使用JACKSON JSON而衍生出来，因为官网上建议将ObjectMapper作为全局变量使用从而提高效率，所以，我们项目里面使用了单例，在使用单例的时候，我们无可厚非的考虑了资源在使用时是否要保证互斥的情况。

最开始的写法：

Java代码

public final class JacksonJsonMapper {

static volatile ObjectMapper objectMapper = null;

private JacksonJsonMapper(){}

public static ObjectMapper getInstance(){

if (objectMapper==null){

objectMapper = new ObjectMapper();

}

return objectMapper;

}

}

在此期间，我考虑了两个问题，并与团队中的另外一个兄弟发生了激烈的讨论：

1、在使用getInstance()方法的时候，是否要使用synchronized关键字。

2、在使用objectMapper.writeValueAsString(object)时，因为此方法非静态方法，在此方法内是否会使用到对象自有的属性，而在并发的时候出现前者属性被后者覆盖的问题。

后再看了源码后，排除了第二个顾虑，ObjectMapper是与线程绑定的，所以是线程安全的，并且也在官网的线程安全介绍中得到了证实。

Jackson follows thread-safety rules typical for modern factory-based Java data format handlers (similar to what, say, Stax or JAXP implementations do). For example:

Factories (ObjectMapper, JsonFactory) are thread-safe once configured: so ensure that all configuration is done from a single thread, and before instantiating anything with factory.

Reader/writer instances (like JsonParser and JsonParser) are not thread-safe -- there is usually no need for them to be, but if for some reason you need to access them from multiple threads, external synchronization is needed

All transformer objects (custom serializers, deserializers) are expected to be stateless, and thereby thread safe -- state has to be stored somewhere outside instances (in ThreadLocal or context objects passed in, like DeserializationContext).

第一个顾虑在看完下面这篇文章后，得到了解决方法：

Java代码

public final class JacksonJsonMapper {

static volatile ObjectMapper objectMapper = null;

private JacksonJsonMapper(){}

public static ObjectMapper getInstance(){

if (objectMapper==null){

synchronized (ObjectMapper.class) {

if (objectMapper==null){

objectMapper = new ObjectMapper();

}

}

}

return objectMapper;

}

}

文章中详细说明了关键字 volatile 是在读取所申明的对象时，会要从内存中进行同步，但是不会对写时起作用，所以，还是需要synchronized 关键字的配合。