Flow API – Flow

In the previous sections, we discussed the components of the Flow API one by one in depth. They are all interfaces and are defined as static components within another component of the Flow API. This component is Flow.

In the Java 9 Flow API, this Flow component contains the rest of the four components' static components, as shown here:

Flow.java:

package java.util.concurrent; 
 
public final class Flow { 
 
    private Flow() {}  
   
    @FunctionalInterface 
    public static interface Publisher<T> { 
        public void subscribe(Subscriber<? super T> subscriber); 
    } 
 
    public static interface Subscriber<T> { 
 
        public void onSubscribe(Subscription subscription); 
 
        public void onNext(T item); 
 
        public void onError(Throwable throwable); 
 
        public void onComplete(); 
    } 
 
    public static interface Subscription { 
 
        public void request(long n); 
 
        public void cancel(); 
    } 
 
    public static interface Processor<T,R> extends Subscriber<T>,
     Publisher<R> { 
     } 
 
    static final int DEFAULT_BUFFER_SIZE = 256; 
 
    public static int defaultBufferSize() { 
        return DEFAULT_BUFFER_SIZE; 
    } 
 
} 

This is so that we can access other components as Flow.xxxx, which means if we want to access a Publisher, we should use it like Flow.Publisher.

When we combine or connect all these components in a working system, we will see them as follows:

When we connect the Flow API components in this way, we can observe that a flow is going from source to destination. That's why they have named this API as the Flow API.

We can represent the Java 9 Flow API's Publisher/Subscriber complete communication as shown in the following diagram. This communication can end either successfully or in failure: