RxJava中常见的Subject有4种，分别是 AsyncSubject、 BehaviorSubject、PublishSubject、 ReplaySubject。

AsyncSubject

　　使用AsyncSubject无论输入多少参数，永远只输出最后一个参数。

BehaviorSubject

　　BehaviorSubject会发送离订阅最近的上一个值，没有上一个值的时候会发送默认值。

BehaviorSubject
     
       subject = BehaviorSubject.create();        subject.onNext(1);        subject.onNext(2);        subject.onNext(3);        subject.onNext(4);        subject.subscribe(new Consumer
      
       (){            @Override            public void accept(Integer t) throws Exception {                System.out.println(t);                            }});
      
     

PublishSubject

　　使用PublishSubject从那里订阅就从那里开始发送数据。

PublishSubject
     
       subject = PublishSubject.create();        subject.onNext(1);        subject.subscribe(new Consumer
      
       (){            @Override            public void accept(Integer t) throws Exception {                System.out.println(t);                            }});        subject.onNext(2);        subject.onNext(3);        subject.onNext(4);
      
     

　　输出结果2 3 4.

ReplaySubject

　　无论何时订阅，都会将所有历史订阅内容全部发出。

ReplaySubject
     
       subject = ReplaySubject.create();        subject.onNext(1);        subject.subscribe(new Consumer
      
       (){            @Override            public void accept(Integer t) throws Exception {                System.out.println(t);                            }});        subject.onNext(2);        subject.onNext(3);        subject.onNext(4);
      
     

　　输出结果1 2 3 4.