关于SimpleDateFormat

• SimpleDateFormat这个类本身是线程不安全的；
• 对于它的使用，要作为局部变量，以堆栈封闭的方式，线程安全的使用 ；

关于joda-time相关类

• 这个包中提供的时间相关类是线程安全的，其依赖为：

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.9</version>
</dependency>

SimpleDateFormat示例 - 反例

• 将SimpleDateFormat示例作为共享变量使用，会暴露出其线程不安全的性质；

import com.example.concurrency.annotations.NotThreadSafe;
import lombok.extern.slf4j.Slf4j;

import java.text.SimpleDateFormat;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

@Slf4j
@NotThreadSafe
public class DateFormatExample1 {

    private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update() {
        try {
            simpleDateFormat.parse("20180208");
        } catch (Exception e) {
            log.error("parse exception", e);
        }
    }
}

输出：

• 会由于其线程不安全的属性造成很多异常；

17:31:36.869 [pool-1-thread-344] ERROR com.example.concurrency.example.commonunsafe.DateFormatExample1 - parse exception
java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Long.parseLong(Long.java:601)
at java.lang.Long.parseLong(Long.java:631)
at java.text.DigitList.getLong(DigitList.java:195)
at java.text.DecimalFormat.parse(DecimalFormat.java:2051)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.example.concurrency.example.commonunsafe.DateFormatExample1.update(DateFormatExample1.java:46)
at com.example.concurrency.example.commonunsafe.DateFormatExample1.lambda0(DateFormatExample1.java:32)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

SimpleDateFormat示例 - 正例

• 将其示例作为局部变量使用，将不会暴露出其线程不安全性；

import com.example.concurrency.annotations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;

import java.text.SimpleDateFormat;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

@Slf4j
@ThreadSafe
public class DateFormatExample2 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update() {
        try {
            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
            simpleDateFormat.parse("20180208");
        } catch (Exception e) {
            log.error("parse exception", e);
        }
    }
}

输出：

• 没有抛出异常；

joda-time相关类示例

• 实际使用时更推荐用joda-time相关类处理时间问题；

import com.example.concurrency.annotations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

@Slf4j
@ThreadSafe
public class DateFormatExample3 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyyMMdd");

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal; i++) {
            final int count = i;
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update(count);
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update(int i) {
        log.info("{}, {}", i, DateTime.parse("20180208", dateTimeFormatter).toDate());
    }
}

输出：

17:39:42.308 [pool-1-thread-1] INFO com.example.concurrency.example.commonunsafe.DateFormatExample3 - 0, Thu Feb 08 00:00:00 CST 2018
...
17:39:42.517 [pool-1-thread-1636] INFO com.example.concurrency.example.commonunsafe.DateFormatExample3 - 4999, Thu Feb 08 00:00:00 CST 2018