java - 即使函数签名抛出它，编译器也会提示未处理的 IOException

Question

我有一个类，其中静态工厂方法使用 computeIfAbsent()返回一个实例:

public class TrafficMonitor {
   private static ConcurrentMap<String, TrafficMonitor> instances = new ConcurrentHashMap<>();
   private String busNumber;

   //private constructor could throw an IOException
   private TrafficMonitor(String busNumber) throws IOException {
       this.busNumber = busNumber;
       // it needs to be in constructor because it call a 3rd party API to instantiate an object which throws IOException
       doIoOperation();
   }

   // static factory method to return a instance of this class
   public static TrafficMonitor forBus(String busNumber) throws IOException {
       // Compiler error: Unhandled exception: java.io.IOException
       return instances.computeIfAbsent(busNumber, TrafficMonitor::new);
   }

}

静态工厂函数中私有(private)构造函数抛出 IOException forBus(String busNumber)即使我有 throws IOException在签名中，编译器仍然提示 unhandled exception: java.io.IOException 。为什么？如何摆脱它？

另一个问题:在这个例子中， TrafficMonitor::new 是吗？自动获取参数busNumber来自forBus(String busNumber)函数或者它如何使用需要参数的私有(private)构造函数创建实例 String busNumber ？

Answer 1

computeIfAbsent 需要一个 Function 作为参数。 Function的SAM的签名是R apply(T t)。如您所见，Function 可能不会抛出 IOException。但您的构造函数确实会抛出 IOException。所以 TrafficMonitor::new 不是一个函数。因此，您不能将其作为参数传递给 computeIfAbsent()。

干净的方式:不要在构造函数中执行 IO:

public class TrafficMonitor {
    private static ConcurrentMap<String, TrafficMonitor> instances = new ConcurrentHashMap<>();
    private String busNumber;
    private boolean initialized;

    private TrafficMonitor(String busNumber) {
        this.busNumber = busNumber;
    }

    private synchronized void initialize() throws IOException {
        if (!initialized) {
            doIoOperation();
            initialized = true;
        }
    }

    public static TrafficMonitor forBus(String busNumber) throws IOException {
        TrafficMonitor result = instances.computeIfAbsent(busNumber, TrafficMonitor::new);
        result.initialize();
        return result;
   }
}

不太干净的方法:将 IOException 作为运行时异常重新抛出

private TrafficMonitor(String busNumber) {
    this.busNumber = busNumber;
    try { 
        doIoOperation();
    } catch(IOException e) {
        throw UnchedkIOException(e);
    }
}

或

public static TrafficMonitor forBus(String busNumber) {
    return instances.computeIfAbsent(busNumber, busNumber -> {
        try {
            return new TrafficMonitor(busNumber);
        } catch(IOException e) {
            throw UnchedkIOException(e);
        }
    });
}