问题介绍

在日常的开发过程中，我们不可避免地会使用到 JDK8 之前的 Date 类，在格式化日期或解析日期时就需要用到 SimpleDateFormat 类，但由于该类并不是线程安全的，所以我们常发现对该类的不恰当使用会导致日期解析异常，从而影响线上服务可用率。

以下是对 SimpleDateFormat 类不恰当使用的示例代码：

package com.jd.threadsafe;

import java.text.SimpleDateFormat;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * @Date: 2023/7/25 10:47
 * @Desc: SimpleDateFormat 线程安全问题复现
 * @Version: V1.0
 **/
public class SimpleDateFormatTest {
    private static final AtomicBoolean STOP = new AtomicBoolean();
    private static final SimpleDateFormat FORMATTER = new SimpleDateFormat("yyyy-M-d"); // 非线程安全

    public static void main(String[] args) {
        Runnable runnable = () -> {
            int count = 0;
            while (!STOP.get()) {
                try {
                    FORMATTER.parse("2023-7-15");
                } catch (Exception e) {
                    e.printStackTrace();
                    if (++count > 3) {
                        STOP.set(true);
                    }
                }
            }
        };

        new Thread(runnable).start();
        new Thread(runnable).start();
    }
}

以上代码模拟了多线程并发使用 SimpleDateFormat 实例的场景，此时可观察到如下异常输出：

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:2082)
	at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
	at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
	at java.text.DateFormat.parse(DateFormat.java:364)
	at com.jd.threadsafe.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:21)
	at java.lang.Thread.run(Thread.java:750)
java.lang.NumberFormatException: For input string: ""
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Long.parseLong(Long.java:601)
	at java.lang.Long.parseLong(Long.java:631)
	at java.text.DigitList.getLong(DigitList.java:195)
	at java.text.DecimalFormat.parse(DecimalFormat.java:2082)
	at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
	at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
	at java.text.DateFormat.parse(DateFormat.java:364)
	at com.jd.threadsafe.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:21)
	at java.lang.Thread.run(Thread.java:750)
java.lang.NumberFormatException: multiple points
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)
	at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
	at java.lang.Double.parseDouble(Double.java:538)
	at java.text.DigitList.getDouble(DigitList.java:169)
	at java.text.DecimalFormat.parse(DecimalFormat.java:2087)
	at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
	at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
	at java.text.DateFormat.parse(DateFormat.java:364)
	at com.jd.threadsafe.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:21)
	at java.lang.Thread.run(Thread.java:750)
java.lang.NumberFormatException: multiple points
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)
	at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.