后端

Solr删除或清空索引

删除索引

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<delete><id>1</id></delete>
<commit/>

或

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<delete><query>id:1</query></delete>
<commit/>

清空索引

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<delete><query>*:*</query></delete>
<commit/>

时间复杂度

常见的时间复杂度有：常数阶O(1),对数阶O(log2n),线性阶O(n), 线性对数阶O(nlog2n),平方阶O(n2)，立方阶O(n3),…， k次方阶O(nk),指数阶O(2n)。

Solr获取document的score及组合排序

获取每个document的score

查询是时需要将score作为返回字段

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fl=*,score

先根据具体字段再根据score排序

Solr默认是按照score排序的，如果有下面需求，需要先按照某个字段排序（如类型），再按照score排序则需要传值

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sort=字段名 ASC,score DESC

获取当前Java进程的活动线程数

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/**
 * 获取当前Java进程的活动线程数
 * @return
 */
public Map<String, Object> getThreadsTotal() {
    Map<String, Object> threadDataMap = new HashMap<String, Object>();

    Integer currentThreadCount = 0;
    Integer currentThreadsBusy = 0;

    MBeanServer mbeanServer = ManagementFactory.getPlatformMBeanServer();
    List<ObjectName> threadPools = new ArrayList<ObjectName>();
    try {
        threadPools.addAll(mbeanServer.queryNames(new ObjectName("*:type=ThreadPool,*"), null));

        for (final ObjectName threadPool : threadPools) {
            //获取所有线程池下的线程总数
            currentThreadCount += (Integer) mbeanServer.getAttribute(threadPool, "currentThreadCount");
            currentThreadsBusy += (Integer) mbeanServer.getAttribute(threadPool, "currentThreadsBusy");

        }
    } catch (MalformedObjectNameException e) {
        logger.error("Get threads information error.", e);
    } catch (ReflectionException e) {
        logger.error("Reflecttion error", e);
    } catch (InstanceNotFoundException e) {
        logger.error("Instance not found error.", e);
    } catch (MBeanException e) {
        logger.error("Mean error", e);
    } catch (AttributeNotFoundException e) {
        logger.error("Could not get attribute", e);
    }

    threadDataMap.put("currentThreadCount", currentThreadCount);
    threadDataMap.put("currentThreadsBusy", currentThreadsBusy);

    return threadDataMap;
}

获得Tomcat的内存信息

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/**
 * 获得Tomcat的内存信息
 * @return
 */
private Map<String, Object> getMemoryInfo() {
    Map<String, Object> memDataMap = new HashMap<String, Object>();

    double MB = 1024.0 * 1024.0;

    //当前JVM的最大可用内存
    double maxMemory = (Runtime.getRuntime().maxMemory()) / MB;

    //当前JVM占用的内存总数
    double totalMemory = (Runtime.getRuntime().totalMemory()) / MB;

    //因为JVM只有在需要内存时才占用物理内存使用，所以freeMemory()的值一般情况下都很小
    double freeMemory = (Runtime.getRuntime().freeMemory()) / MB;

    //JVM实际可用内存
    double totalUsableMemory = maxMemory - totalMemory + freeMemory;

    memDataMap.put("totalUsableMemory", totalUsableMemory);
    memDataMap.put("totalMemory", totalMemory);
    memDataMap.put("maxMemory", maxMemory);

    NumberFormat nt = NumberFormat.getPercentInstance();
    nt.setMinimumFractionDigits(2);
    memDataMap.put("memUsage", nt.format(totalMemory / maxMemory));

    return memDataMap;
}

tomcat启动不报错但是服务也没有启动成功

之前遇到了一次tomcat启动时没有报任何错误，但是服务也没有启动成功，访问不了，后来同事帮忙在网上找到这样一篇文章，这里记录下
DEBUGGING THE DREADED “SEVERE: ERROR LISTENERSTART” AND “SEVERE: ERROR FILTERSTART” TOMCAT ERROR MESSAGES

具体操作：

  1. 中web application下的WEB-INF/classes路径中创建logging.properties文件，其内容为：

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    org.apache.catalina.core.ContainerBase.[Catalina].level = INFO //INFO级别可以更改
org.apache.catalina.core.ContainerBase.[Catalina].handlers = java.util.logging.ConsoleHandler

  2. 启动服务，这个时候错误就会打印到控制台（catalina.out）