Dubbo: 这种写法都是为了啥？为什么不通过putlfabsent时直接获取呢

Created on 13 Sep 2017 · 9Comments · Source: apache/dubbo

@SuppressWarnings("unchecked")
private T createExtension(String name) {
    Class<?> clazz = getExtensionClasses().get(name);
    if (clazz == null) {
        throw findException(name);
    }
    try {
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) {
           // ************************************************************ //
           EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
           instance = (T) EXTENSION_INSTANCES.get(clazz);
           // ************************************************************ //
        }
        injectExtension(instance);
        Set<Class<?>> wrapperClasses = cachedWrapperClasses;
        if (wrapperClasses != null && wrapperClasses.size() > 0) {
            for (Class<?> wrapperClass : wrapperClasses) {
                instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
            }
        }
        return instance;
    } catch (Throwable t) {
        throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
                type + ")  could not be instantiated: " + t.getMessage(), t);
    }
}

typquestion & discussion

Source

debugmoney

All 9 comments

问题描述不清不楚

bigcai on 13 Sep 2017

❤1

EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);

这句话为什么总分开写呢，我发现里面有很多这种写法，为什么不通过这种呢

instance = EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());

debugmoney on 13 Sep 2017

看起来舒服吧，而且彼此间不影响，用起来也舒服

lllichen on 13 Sep 2017

😕1

@debugmoney
在java8之前这么分开写是最优的，为了降低程序计算量，避免(T) clazz.newInstance()每次都执行，这也是java不支持函数式惰性求值的弊端.

在java8 之后可以通过computeIfAbsent方法来解决，你可以去了解下java8的computeIfAbsent方法，了解后，这个问题就很清楚了。

java8之后可以这么写：

EXTENSION_INSTANCES.computeIfAbsent(clazz, clazz -> (T) clazz.newInstance());

wuwen5 on 13 Sep 2017

👍1

@debugmoney 你给的写法，结果是错的。

PS: 有想法要自己先验证一下正确与否哦～ 😓 如果脑子推理不清，就写个测试/Demo的代码来验证。
更多信息可以看ConcurrentHashMap#putIfAbsent的JavaDoc

验证代码如下：

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

public class Test {
    public static void main(String[] args) {
        final ConcurrentMap<String, String> map11 = new ConcurrentHashMap<String, String>();

        System.out.println(map11.putIfAbsent("key1", "v1")); // output null ! Wrong !!
        System.out.println(map11.putIfAbsent("key1", "v2")); // output v1
        System.out.println(map11.putIfAbsent("key1", "v3")); // output v1

        // Output is: (Wrong value, Bug!)
        // null
        // v1
        // v1

        final ConcurrentMap<String, String> map22 = new ConcurrentHashMap<String, String>();

        map22.putIfAbsent("key1", "v1");
        System.out.println(map22.get("key1")); // output v1 , Right.
        map22.putIfAbsent("key1", "v2");
        System.out.println(map22.get("key1")); // output v1
        map22.putIfAbsent("key1", "v3");
        System.out.println(map22.get("key1")); // output v1

        // Output is:
        // v1
        // v1
        // v1
    }
}

另外， @wuwen5