Angular2获取请求两次

我的服务看起来像这样：

getProjects(): Observable<Project[]> {
    return this.http.get(thiS.Url)
        .map(this.mapProjects)
        .catch(this.handleError);
}

 private mapProjects(response: ResponsE): any {
    const mappedProjects = response.json();
    return mappedProjects;
}

我的组件看起来像这样：

export class ProjectListComponent implements OnInit {

// List of projects
listProjects: Project[] = [];

constructor(private projectListservice: ProjectListservicE) {
 }

public getProjectList() {
    this.projectListservice.getProjects()
        .subscribe(
        projects => {
            this.listProjects = projects;
        },error => {
            // error handling
        });
}
}

在Chrome开发者工具的网络标签中,我看到请求是两次,一旦启动器是zone.js,另一次只是说“其他”.谁能解释这种行为？