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工具篇：介绍几个好用的guava工具类

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前言

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平时我们都会封装一些处理缓存或其他的小工具。但每个人都封装一次，重复造轮子，有点费时间。有没有一些好的工具库推荐-guava。guava是谷歌基于java封装好的开源库，它的性能、实用性，比我们自己造的轮子更好，毕竟谷歌出品，下面介绍下几个常用的guava工具类

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guava的maven配置引入

<dependency>
　　<groupId>com.google.guava</groupId>
　　<artifactId>guava</artifactId>
　　<version>27.0-jre</version>
 </dependency>

LoadingCache

CacheBuilder 方法参数描述
initialCapacity(int initialCapacity)缓存池的初始大小
concurrencyLevel(int concurrencyLevel)设置并发数
maximumSize(long maximumSize)缓存池大小，在缓存项接近该大小时， Guava开始回收旧的缓存项
weakValues()设置value的存储引用是虚引用
softValues()设置value的存储引用是软引用
expireAfterWrite(long duration, TimeUnit unit)设置时间对象没有被写则对象从内存中删除(在另外的线程里面不定期维护)
expireAfterAccess(long duration, TimeUnit unit)设置时间对象没有被读/写访问则对象从内存中删除(在另外的线程里面不定期维护)
refreshAfterWrite(long duration, TimeUnit unit)和expireAfterWrite类似，不过不立马移除key，而是在下次更新时刷新，这段时间可能会返回旧值
removalListener( RemovalListener<? super K1, ? super V1> listener)监听器，缓存项被移除时会触发
build(CacheLoader<? super K1, V1> loader)当数据不存在时，则使用loader加载数据
LoadingCache<Integer,Long> cacheMap = CacheBuilder.newBuilder().initialCapacity(10)
    .concurrencyLevel(10)
    .expireAfterAccess(Duration.ofSeconds(10))
    .weakValues()
    .recordStats()
    .removalListener(new RemovalListener<Integer,Long>(){
        @Override
        public void onRemoval(RemovalNotification<Integer, Long> notification) {
            System.out.println(notification.getValue());
        }
    })
    .build(new CacheLoader<Integer,Long>(){
        @Override
        public Long load(Integer key) throws Exception {
            return System.currentTimeMillis();
        }
    });
cacheMap.get(1);

Multimap 和 MultiSet

//Multimap: key-value  key可以重复，value也可重复
Multimap<String, String> multimap = ArrayListMultimap.create();
multimap.put("csc","1");
multimap.put("lwl","1");
multimap.put("csc","1");
multimap.put("lwl","one");
System.out.println(multimap.get("csc"));
System.out.println(multimap.get("lwl"));
---------------------------
[1, 1]
[1, one]

//MultiSet: 无序+可重复   count()方法获取单词的次数  增强了可读性+操作简单
Multiset<String> set = HashMultiset.create();
set.add("csc");
set.add("lwl");
set.add("csc");
System.out.println(set.size());
System.out.println(set.count("csc"));
---------------------------
3
2

BiMap

BiMap<Integer,String> biMap = HashBiMap.create();
biMap.put(1,"lwl");
biMap.put(2,"csc");
BiMap<String, Integer> map = biMap.inverse(); // value和key互转
map.forEach((v, k) -> System.out.println(v + "-" + k));

Table

// 双键的Map Map--> Table-->rowKey+columnKey+value  
Table<String, String, Integer> tables = HashBasedTable.create();
tables.put("csc", "lwl", 1);
//row+column对应的value
System.out.println(tables.get("csc","lwl"));

Sets和Maps

// 不可变集合的创建
ImmutableList<String> iList = ImmutableList.of("csc", "lwl");
ImmutableSet<String> iSet = ImmutableSet.of("csc", "lwl");
ImmutableMap<String, String> iMap = ImmutableMap.of("csc", "hello", "lwl", "world");

set的交集, 并集, 差集

HashSet setA = newHashSet(1, 2, 3, 4, 5);  
HashSet setB = newHashSet(4, 5, 6, 7, 8); 
//并集
SetView union = Sets.union(setA, setB);   
//差集 setA-setB
SetView difference = Sets.difference(setA, setB);  
//交集
SetView intersection = Sets.intersection(setA, setB);

map的交集，并集，差集

HashMap<String, Integer> mapA = Maps.newHashMap();
mapA.put("a", 1);mapA.put("b", 2);mapA.put("c", 3);
HashMap<String, Integer> mapB = Maps.newHashMap();
mapB.put("b", 20);mapB.put("c", 3);mapB.put("d", 4);
MapDifference<String, Integer> mapDifference = Maps.difference(mapA, mapB);
//mapA 和 mapB 相同的 entry
System.out.println(mapDifference.entriesInCommon());
//mapA 和 mapB key相同的value不同的 entry
System.out.println(mapDifference.entriesDiffering());
//只存在 mapA 的 entry
System.out.println(mapDifference.entriesOnlyOnLeft());
//只存在 mapB 的 entry
System.out.println(mapDifference.entriesOnlyOnRight());;
-------------结果-------------
{c=3}
{b=(2, 20)}
{a=1}
{d=4}

EventBus

@Data
@AllArgsConstructor
public class OrderMessage {
    String message;
}
//使用 @Subscribe 注解,表明使用dealWithEvent 方法处理 OrderMessage类型对应的消息
//可以注解多个方法,不同的方法 处理不同的对象消息
public class OrderEventListener {
    @Subscribe
    public void dealWithEvent(OrderMessage event) {
        System.out.println("内容：" + event.getMessage());
    }
}
-------------------------------------
// new AsyncEventBus(String identifier, Executor executor);
EventBus eventBus = new EventBus("lwl"); 
eventBus.register(new OrderEventListener());
// 发布消息
eventBus.post(new OrderMessage("csc"));

StopWatch

Stopwatch stopwatch = Stopwatch.createStarted();
for(int i=0; i<100000; i++){
    // do some thing
}
long nanos = stopwatch.elapsed(TimeUnit.MILLISECONDS);
System.out.println("逻辑代码运行耗时："+nanos);

Files文件操作

File newFile = new File("D:/text.txt");
Files.write("this is a test".getBytes(), newFile);
//再次写入会把之前的内容冲掉
Files.write("csc".getBytes(), newFile);
//追加写
Files.append("lwl", newFile, Charset.defaultCharset());

File newFile = new File("E:/text.txt");
List<String> lines = Files.readLines(newFile, Charset.defaultCharset());
方法描述
Files.copy(File from, File to)复制文件
Files.deleteDirectoryContents(File directory)删除文件夹下的内容(包括文件与子文件夹)
Files.deleteRecursively(File file)删除文件或者文件夹
Files.move(File from, File to)移动文件
Files.touch(File file)创建或者更新文件的时间戳
Files.getFileExtension(String file)获得文件的扩展名
Files.getNameWithoutExtension(String file)获得不带扩展名的文件名
Files.map(File file, MapMode mode)获取内存映射buffer

RateLimiter

//RateLimiter 构造方法，每秒限流permitsPerSecond
public static RateLimiter create(double permitsPerSecond) 
//每秒限流 permitsPerSecond，warmupPeriod 则是数据初始预热时间，从第一次acquire 或 tryAcquire 执行开时计算
public static RateLimiter create(double permitsPerSecond, Duration warmupPeriod)
//获取一个令牌，阻塞，返回阻塞时间
public double acquire()
//获取 permits 个令牌，阻塞，返回阻塞时间
public double acquire(int permits)
//获取一个令牌，超时返回
public boolean tryAcquire(Duration timeout)
获取 permits 个令牌，超时返回
public boolean tryAcquire(int permits, Duration timeout)

RateLimiter limiter = RateLimiter.create(2, 3, TimeUnit.SECONDS);
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
System.out.println("get one permit cost time: " + limiter.acquire(1) + "s");
---------------  结果 -------------------------
get one permit cost time: 0.0s
get one permit cost time: 1.331672s
get one permit cost time: 0.998392s
get one permit cost time: 0.666014s
get one permit cost time: 0.498514s
get one permit cost time: 0.498918s
get one permit cost time: 0.499151s
get one permit cost time: 0.488548s

Guava Retry

<dependency>
  <groupId>com.github.rholder</groupId>
  <artifactId>guava-retrying</artifactId>
  <version>2.0.0</version>
</dependency>
RetryerBuilder方法描述
withRetryListener重试监听器
withWaitStrategy失败后重试间隔时间
withStopStrategy停止策略
withBlockStrategy阻塞策略BlockStrategy
withAttemptTimeLimiter执行时间限制策略
retryIfException发生异常，则重试
retryIfRuntimeException发生RuntimeException异常，则重试
retryIfExceptionOfType(Class<? extends Throwable> ex)发生ex异常，则重试
retryIfException(Predicate<Throwable> exceptionPredicate)对异常判断，是否重试
retryIfResult(Predicate<V> resultPredicate)对返回结果判断，是否重试
Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
    .retryIfException()
    .retryIfResult(Predicates.equalTo(false))
    .withAttemptTimeLimiter(AttemptTimeLimiters.fixedTimeLimit(1, TimeUnit.SECONDS))
    .withStopStrategy(StopStrategies.stopAfterAttempt(5))
    .build();
//Retryer调用                
retryer.call(() -> true);

参考文章

标签：get,System,cost,permit,println,工具,guava,好用,out
来源： https://blog.csdn.net/nidongla/article/details/118027201