2-《Java并发编程实战》(Java Concurrency in Practice) 代码示例

说明

        这是针对《Java并发编程实战》(Java Concurrency in Practice)一书中的示例代码进行扩展，并且进行验证的完整代码，具体背景可看这篇文章：1-《Java并发编程实战》(Java Concurrency in Practice) 代码示例

        下面的示例代码都是针对书中的（不完整的）代码扩展，不再介绍具体上下文背景（如果你也在看这本书，可以拿我这里的示例代码做个参考，并欢迎提出意见和建议）

程序清单2-5

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.atomic.AtomicReference;

/**
 * @DESCRIPTION:
 * @USER: shg
 * @DATE: 2024/1/13 11:15
 */
public class UnsafeCachingFactorizer extends HttpServlet {

    private final AtomicReference lastNumber = new AtomicReference<>();
    private final AtomicReference lastFactors = new AtomicReference<>();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        BigInteger i = extractFromRequest(req);
        if (i.equals(lastNumber.get())) {
            encodeIntoResponse(resp, i, lastFactors.get());
        } else {
            BigInteger[] factors = factor(i);
            lastNumber.set(i);
            lastFactors.set(factors);
            encodeIntoResponse(resp, i, lastFactors.get());
        }
    }

    private BigInteger extractFromRequest(HttpServletRequest req) {
        return new BigInteger(req.getParameter("key"));
    }

    private void encodeIntoResponse(HttpServletResponse resp, BigInteger i, BigInteger[] factors) throws IOException {
        System.out.println(i + "--->" + Arrays.asList(factors));
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        writer.println("");
        writer.println("
" + i + "因数分解结果为：" + Arrays.asList(factors));
        writer.println("");
    }

    private BigInteger[] factor(BigInteger number) {
        List factors = new ArrayList<>();
        BigInteger divisor = BigInteger.valueOf(2);
        while (number.compareTo(BigInteger.ONE) > 0) {
            if (number.mod(divisor).equals(BigInteger.ZERO)) {
                number = number.divide(divisor);
                factors.add(divisor);
            } else {
                divisor = divisor.add(BigInteger.ONE);
            }
        }
        return factors.toArray(new BigInteger[0]);
    }
}

压测线程设置

        使用Jmeter开启两个线程组，每个线程组传递的参数不一样，但是访问同一个Servlet组件（即：访问同一个UnsafeCachingFactorizer类），然后在控制台分析输出结果，具体操作如下图：

 ​​​​​​​

 

压测结果分析

         首先要明确的是，这是一个Servlet组件（即：UnsafeCachingFactorizer类）的功能是因数分解，并且希望将最近的计算结果缓存起来，即当两个连续的请求对相同的数值进行因数分解时，可以直接使用上一次的计算结果，而无需重新计算。

        并且我们可以事先知道是的129的因数分解结果应该是 3和43；24的因数分解结果应该是：2，2，2和3，现在看如下图：

程序清单2-6

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @DESCRIPTION: 解决了 UnsafeCachingFactorizer_2_5 并发访问安全性问题，但是性能降低（不能接受）
 * @USER: shg 
 * @DATE: 2024/1/13 12:09
 */
public class SynchronizedFactorizer2_6 extends HttpServlet {

    private BigInteger lastNumber;
    private BigInteger[] lastFactors;

    @Override
    protected synchronized void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        BigInteger i = extractFromRequest(req);
        if (i.equals(lastNumber)) {
            encodeIntoResponse(resp, i, lastFactors);
        } else {
            BigInteger[] factors = factor(i);
            lastNumber = i;
            lastFactors = factors;
            encodeIntoResponse(resp, i, factors);
        }
    }

    private BigInteger extractFromRequest(HttpServletRequest req) {
        return new BigInteger(req.getParameter("key"));
    }

    private void encodeIntoResponse(HttpServletResponse resp, BigInteger i, BigInteger[] factors) throws IOException {
        System.out.println(i + "--->" + Arrays.asList(factors));
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        writer.println("");
        writer.println("
" + i + "因数分解结果为：" + Arrays.asList(factors));
        writer.println("");
    }

    private BigInteger[] factor(BigInteger number) {
        List factors = new ArrayList<>();
        BigInteger divisor = BigInteger.valueOf(2);
        while (number.compareTo(BigInteger.ONE) > 0) {
            if (number.mod(divisor).equals(BigInteger.ZERO)) {
                number = number.divide(divisor);
                factors.add(divisor);
            } else {
                divisor = divisor.add(BigInteger.ONE);
            }
        }
        return factors.toArray(new BigInteger[0]);
    }
}

程序清单2-8

import lombok.extern.slf4j.Slf4j;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * @DESCRIPTION:
 * @USER: shg
 * @DATE: 2024/1/13 12:44
 */
@Slf4j
public class CachedFactorizer_2_8 extends HttpServlet {

    private BigInteger lastNumber;
    private BigInteger[] lastFactors;
    private long hits;
    private long cacheHits;

    public synchronized long getHits() {
        return hits;
    }

    private synchronized long getCacheHits() {
        return cacheHits;
    }

    public synchronized double getCacheHitRatio() {
        return (double) cacheHits / (double) hits;
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        BigInteger i = extractFromRequest(req);
        BigInteger[] factors = null;

        synchronized (this) {
            ++hits;
            if (i.equals(lastNumber)) {
                ++cacheHits;
                factors = lastFactors.clone();
            }
        }

        if (factors == null) {
            factors = factor(i);
            synchronized (this) {
                lastNumber = i;
                lastFactors = factors.clone();
            }
        }
        encodeIntoResponse(resp, i, factors);
    }

    private BigInteger extractFromRequest(HttpServletRequest req) {
        return new BigInteger(req.getParameter("key"));
    }

    private BigInteger[] factor(BigInteger number) {
        List factors = new ArrayList<>();
        BigInteger divisor = BigInteger.valueOf(2);
        while (number.compareTo(BigInteger.ONE) > 0) {
            if (number.mod(divisor).equals(BigInteger.ZERO)) {
                number = number.divide(divisor);
                factors.add(divisor);
            } else {
                divisor = divisor.add(BigInteger.ONE);
            }
        }
        return factors.toArray(new BigInteger[0]);
    }

    private void encodeIntoResponse(HttpServletResponse resp, BigInteger i, BigInteger[] factors) throws IOException {
        System.out.println(i + "--->" + Arrays.asList(factors));
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter writer = resp.getWriter();
        writer.println("");
        writer.println("
" + i + "因数分解结果为：" + Arrays.asList(factors) + "\n");
        writer.println("
 统计请求数量：" + getHits() + "
");
        writer.println("
 缓存命中数量：" + getCacheHits() + "
");
        writer.println("
 缓存命中率：" + getCacheHitRatio() + "
");
        writer.println("");
    }
}