Java有用代码片段

[TFdream]

json序列化

1.google gson

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.5</version>
</dependency>

2.Gson对象序列化

private static final Gson GSON = new GsonBuilder().setDateFormat("yyyy-MM-dd HH:mm:ss").create();

String jsonString = gson.toJson(obj);  

3.Gson对象反序列化

//1.parse object
Person person = gson.fromJson(jsonString, Person.class);  
//2.parse array
List<Person> list = gson.fromJson(jsonString, new TypeToken<List<Person>>(){}.getType());  

4.Gson JsonSerializer 与 JsonDeserializer

例如 针对Date 序列化与反序列化，如下：

import com.google.gson.*;
import org.apache.commons.lang3.StringUtils;

import java.lang.reflect.Type;
import java.util.Date;

/**
 * @author Ricky Fung
 */
public class JsonUtils {
    private static final Gson GSON = new GsonBuilder()
            .registerTypeAdapter(Date.class, new JsonDateSerializer())
            .create();

    public static String toJson(Object obj) {
        return GSON.toJson(obj);
    }

    public static <T> T parseObject(String json, Class<T> classOfT) {
        return GSON.fromJson(json, classOfT);
    }
    public static <T> T parseObject(String json, Type typeOfT) {
        return GSON.fromJson(json, typeOfT);
    }
}

/**
 * gson Date序列化/反序列化
 * @author Ricky Fung
 */
class JsonDateSerializer implements
        JsonSerializer<Date>,JsonDeserializer<Date> {

    @Override
    public Date deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        if (json==null || json.isJsonNull()) {
            return null;
        }
        String dateStr = json.getAsString();
        if (StringUtils.isEmpty(dateStr)) {
            return null;
        }
        return DateUtils.parseDate(dateStr);
    }

    @Override
    public JsonElement serialize(Date src, Type typeOfSrc, JsonSerializationContext context) {
        if (src==null) {
            return null;
        }
        return new JsonPrimitive(DateUtils.format(src));
    }
}

Cache

1.google guava

Guava: Google Core Libraries for Java.

1.guava

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>23.0</version>
</dependency>

2.quick started

LoadingCache<K, V> caches = CacheBuilder.newBuilder()
                .maximumSize(100)
                .expireAfterWrite(10, TimeUnit.MINUTES)
                .build(new CacheLoader<K, V>() {
                    @Override
                    public V load(K key) throws Exception {
                        return generateValueByKey(key);
                    }
                });
try {
    caches.get(key);
} catch (ExecutionException e) {
    e.printStackTrace();
}

2. Caffeine

Caffeine is a high performance, near optimal caching library based on Java 8.

1.caffeine

<dependency>
    <groupId>com.github.ben-manes.caffeine</groupId>
    <artifactId>caffeine</artifactId>
    <version>2.6.2</version>
</dependency>

2.quick started

LoadingCache<Key, Graph> graphs = Caffeine.newBuilder()
    .maximumSize(10_000)
    .expireAfterWrite(5, TimeUnit.MINUTES)
    .refreshAfterWrite(1, TimeUnit.MINUTES)
    .build(key -> createExpensiveGraph(key));

获取默认类加载器

摘自Spring框架中的 org.springframework.util.ClassUtils 中更合理的实现方式：

	public static ClassLoader getDefaultClassLoader() {
		ClassLoader cl = null;
		try {
			cl = Thread.currentThread().getContextClassLoader();
		}
		catch (Throwable ex) {
			// Cannot access thread context ClassLoader - falling back...
		}
		if (cl == null) {
			// No thread context class loader -> use class loader of this class.
			cl = ClassUtils.class.getClassLoader();
			if (cl == null) {
				// getClassLoader() returning null indicates the bootstrap ClassLoader
				try {
					cl = ClassLoader.getSystemClassLoader();
				}
				catch (Throwable ex) {
					// Cannot access system ClassLoader - oh well, maybe the caller can live with null...
				}
			}
		}
		return cl;
	}