来看一段代码，代码中有两个打日志的地方，请问哪个会先打印出来？
这里使用的是Guava的CacheLoader
package com.example.demo.config; import com.alibaba.fastjson.JSONObject; import com.google.common.cache.CacheBuilder; import com.google.common.cache.CacheLoader; import com.google.common.cache.LoadingCache; import lombok.extern.slf4j.Slf4j; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.CommandLineRunner; import org.springframework.stereotype.Component; import java.util.HashMap; import java.util.concurrent.TimeUnit; @Slf4j @Component public class CancelTypeCacheMap implements CommandLineRunner { // 默认值 private static HashMap<String, String> DEFAULT_CACHE_MAP = new HashMap<String, String>() { { put("0", "取消1"); put("1", "取消2"); put("3", "取消3"); put("4", "取消4"); } }; private static volatile LoadingCache<String, HashMap<String, String>> cache; private static final String CANCEL_TYPE_DESC_LIST_KEY = "cancelTypeDescList"; @Override public void run(String... args) throws Exception { if (cache == null) { cache = CacheBuilder.newBuilder() .refreshAfterWrite(30, TimeUnit.MINUTES).build(new CacheLoader<String, HashMap<String, String>>() { @Override public HashMap<String, String> load(String key) throws Exception { //第一个log log.info("cancelTypeDesc加载完成:{}",DEFAULT_CACHE_MAP); return DEFAULT_CACHE_MAP; } }); //第二个log log.info("cancelTypeDescLoadingCache is {}",cache); cache.get(CANCEL_TYPE_DESC_LIST_KEY); } } }
当然是第二个先打印，可是我当时没理解为啥是这样，看了看代码有了以下理解：
上面那段，run方法的代码可以写成下面这样：
@Override public void run(String... args) throws Exception { if (cache == null) { CacheLoader<String, HashMap<String, String>> load= new CacheLoader<String, HashMap<String, String>>() { @Override public HashMap<String, String> load(String key) throws Exception { log.info("map加载完成:{}",DEFAULT_CACHE_MAP); return DEFAULT_CACHE_MAP; } }; cache = CacheBuilder.newBuilder() .refreshAfterWrite(30, TimeUnit.MINUTES).build(load); log.info("cancelTypeDescLoadingCache is {}", cache); HashMap<String, String> result= cache.get(CANCEL_TYPE_DESC_LIST_KEY); log.info("result is {}", JSONObject.toJSONString(result)); } }
然后还需要注意的一点在于，cache.get方法，点进去可以看到：
@Override public V get(K key) throws ExecutionException { return localCache.getOrLoad(key); }
也就是说，当已经有数据的时候，会get，当没有数据的时候，就会load，而我们初始化CacheLoader的时候，实现了load方法，那么为什么是这么写的呢？
因为CacheLoader是一个抽象类，load是一个抽象方法，当我们实例化，也就是new它的时候，必须先实现load方法。
public abstract class CacheLoader<K, V> { /** * Computes or retrieves the value corresponding to {@code key}. * * @param key the non-null key whose value should be loaded * @return the value associated with {@code key}; <b>must not be null</b> * @throws Exception if unable to load the result * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is * treated like any other {@code Exception} in all respects except that, when it is caught, * the thread's interrupt status is set */ public abstract V load(K key) throws Exception; }
