为什么我们一般使用的Spring bean是线程安全?
在spring中bean的默认创建scope是singleton的,即创建的对象是单例的,并且当我们使用这些bean时,如我们经常使用的Service、DAO和Controller,大多数情况下都是以类似工具类的形式使用,只是调用这些bean的方法,而不会对bean的属性、状态进行改变,故不存在多线程竞争,即线程安全。
如果我们要对这些bean的属性改变如何保证线程安全?
通过将scope设置为prototype
当将scope设置为prototype时,每次使用该对象是都会重新构造这个对象,故对象的属性(成员变量)都是线程独有的,是线程安全的。不过,这么多对象的创建和销毁会大量占用内存和消耗系统资源,更推荐使用下面的threadlocal。
通过使用ThreadLocal
当同步对象的属性不需要与其他线程共享,只需要保证在自身线程封闭,而不被其他线程修改时使用
Threadloca原理
Demo:
在每次请求中通过threadlocal类共享变量,通过filter、interceptor验证共享成功。在filter中打印threadlocal的值,在interceptor前设置threadlocal的值并打印,在interceptor后删除threadlocal的值,避免内存泄露。(ps:为了讲解方便,故将所有代码发到一个class文件,实现使用时需要分开)
import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
/**
* \* Created with IntelliJ IDEA.
* \* @author: zuzi
* \* Date: 2019-06-11
* \* Time: 下午12:06
* \* Description:
* \
*/
@Slf4j
@SpringBootApplication
public class JavaStudyApplication implements WebMvcConfigurer {
public static void main(String[] args) {
SpringApplication.run(JavaStudyApplication.class, args);
}
public static final ThreadLocal threadLocal = new ThreadLocal<>();
@RestController("thread-local")
class ThreadLocalController {
@PostMapping
public String threadLocalSet() {
threadLocal.set("controller set");
return "设置成功!";
}
@GetMapping
public String threadLocalGet() {
return "获取到的threadLocal:为" + threadLocal.get();
}
}
class HttpFilter implements Filter {
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) servletRequest;
log.info("do filter {} {} {}", Thread.currentThread().getId(), request.getServletPath(), threadLocal.get());
filterChain.doFilter(request, servletResponse);
}
}
class HttpInterceptor extends HandlerInterceptorAdapter {
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
threadLocal.set("interceptor set");
log.info("pre handle , threadLocal:" + threadLocal.get());
return true;
}
@Override
public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
log.info("after completion, threadLocal:" + threadLocal.get());
threadLocal.remove();
}
}
@Bean
public FilterRegistrationBean httpFilter() {
FilterRegistrationBean registrationBean = new FilterRegistrationBean<>();
registrationBean.setFilter(new HttpFilter());
registrationBean.addUrlPatterns("/thread-local/*");
return registrationBean;
}
@Override
public void addInterceptors(InterceptorRegistry registry) {
registry.addInterceptor(new HttpInterceptor()).addPathPatterns("/**");
}
}
复制代码
结果验证
当请求GET /thread-local
时,由于未设置threadlocal filter打印结果为null,在interceptor 前设置了threadlocal,故打印 interceptor set,在controller中也为设置,返回值为interceptor set,interceptor 后同理为 interceptor set。
请求:
日志:
当请求POST /thread-local
时,类似,结果如下:
请求:
日志:
通过加锁
当同步对象的属性在多线程中共享时使用,参考: Java多线程 - 锁机制
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