Oracle Coherence 缓存总是返回新的对象实例

近日在解决一个业务问题的时候，突然发现使用 Oracle Coherence 时，调用 NamedCache 的 get() 方法总是返回新的对象实例。

示例

package org.opoo.cache.coherence;

import java.io.Serializable;
import com.tangosol.net.CacheFactory;
import com.tangosol.net.NamedCache;

public class NamedCacheSample {
    public static void main(String[] args) {
        NamedCache cache = CacheFactory.getCache("sample");

        Object data = new SampleData();
        cache.put("1000", data);

        System.out.println(data);
        System.out.println(cache.get("1000"));
        System.out.println(cache.get("1000"));
        System.out.println(cache.get("1000"));
    }

    public static class SampleData implements Serializable{
        private static final long serialVersionUID = 3353238636748170244L;
    }
}

运行结果

2014-03-17 12:40:12.998/0.234 Oracle Coherence 3.6.1.0 <Info> (thread=main, member=n/a): Loaded operational configuration from "jar:file:/D:/m2.repo/oracle-coherence/coherence/3.6.1.0-b19636/coherence-3.6.1.0-b19636.jar!/tangosol-coherence.xml"
2014-03-17 12:40:13.014/0.250 Oracle Coherence 3.6.1.0 <Info> (thread=main, member=n/a): Loaded operational overrides from "jar:file:/D:/m2.repo/oracle-coherence/coherence/3.6.1.0-b19636/coherence-3.6.1.0-b19636.jar!/tangosol-coherence-override-dev.xml"

Oracle Coherence Version 3.6.1.0 Build 19636
 Grid Edition: Development mode
Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.

org.opoo.cache.coherence.NamedCacheSample$SampleData@1eec35
org.opoo.cache.coherence.NamedCacheSample$SampleData@77ef83
org.opoo.cache.coherence.NamedCacheSample$SampleData@1cb52ae
org.opoo.cache.coherence.NamedCacheSample$SampleData@26d607

可见，缓存的对象和每次获取的对象实例都是不同的。

原因

对象在 Coherence 中进行缓存时，会调用对象序列化（serialize）。在从缓存中读出时会调用反序列化（deserialize），每次调用反序列化都会创建一个新的对象实例。

所以，如果要反序列化单态类的对象实例，就需要实现 readResolve 方法来对返回实例进行替换。

错误的用法

基于 Coherence 上面所述的特点，下面的代码展示了一种错误的用法。

package org.opoo.cache.coherence;

import java.io.Serializable;
import com.tangosol.net.CacheFactory;
import com.tangosol.net.NamedCache;

public class NamedCacheSample2 {
    public static class SampleData implements Serializable{
        private static final long serialVersionUID = 3353238636748170244L;
        public int status;
        public String toString(){return super.toString() + "[status=" + status + "]";}
    }

    public static String KEY = "1000";
    public static String CACHE_NAME = "sample";
    public static void main(String[] args){
        NamedCache cache = CacheFactory.getCache(CACHE_NAME);
        SampleData data = new SampleData();
        cache.put(KEY, data);

        m1(cache);
    }

    public static void m1(NamedCache cache){
        SampleData data1 = (SampleData) cache.get(KEY);
        m2(cache);

        if(data1.status > 0){
            System.out.println("执行业务逻辑");
        }else{
            String string = String.format("业务逻辑未执行\n%s\n%s", data1, cache.get(KEY));
            System.out.println(string);
        }
    }

    public static void m2(NamedCache cache){
        SampleData data2 = (SampleData) cache.get(KEY);
        data2.status = 10;
        //持久化对象
        //saveToDatabase(data2);
        cache.put(KEY, data2);
    }
}

运行结果

业务代码未执行
org.opoo.cache.coherence.NamedCacheSample2$SampleData@15575e0[status=0]
org.opoo.cache.coherence.NamedCacheSample2$SampleData@1addb59[status=10]