SimpleDateFormat在多執行緒中會出現執行緒安全問題。

class DateThread extends Thread {
    private SimpleDateFormat sdf;
    private String dateString;

    public DateThread(SimpleDateFormat sdf, String dateString) {
        this.sdf = sdf;
        this.dateString = dateString;
    }

    @Override
    public void run() {
        try {
            Date date = sdf.parse(dateString);
            String newDateString = sdf.format(date).toString();
            if (!newDateString.equals(dateString)) {
                System.out.println("ThreadName = " + this.getName()
                        + "報錯了 日期字串：" + dateString
                        + " 轉換成的日期為" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}
public class Run3_formatError {
    public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        String[] dateStringArray = new String[] {
                "2018-12-01", "2018-12-02", "2018-12-03", "2018-12-04", "2018-12-05",
                "2018-12-06", "2018-12-07", "2018-12-08", "2018-12-09", "2018-12-10"
        };
        DateThread[] threads = new DateThread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new DateThread(sdf, dateStringArray[i]);
        }

        for (int i = 0; i < 10; i++) {
            threads[i].start();
        }
    }
}
 

報錯：

java.lang.NumberFormatException: For input string: "1212.E12122E2"
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Long.parseLong(Long.java:589)
	at java.lang.Long.parseLong(Long.java:631)
	at java.text.DigitList.getLong(DigitList.java:195)
	at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
	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.brianway.learning.java.multithread.supplement.example3.DateThread.run(Run3_formatError.java:28)
java.lang.NumberFormatException: empty String
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
	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:2089)
	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.brianway.learning.java.multithread.supplement.example3.DateThread.run(Run3_formatError.java:28)
ThreadName = Thread-1報錯了 日期字串：2018-12-02 轉換成的日期為2222-12-01
ThreadName = Thread-0報錯了 日期字串：2018-12-01 轉換成的日期為2222-12-01
 

解決：

class DateThread extends Thread {
    private SimpleDateFormat sdf;
    private String dateString;
    //通過本地執行緒池 解決執行緒安全問題
    private static ThreadLocal<SimpleDateFormat> tl = new ThreadLocal<SimpleDateFormat>();
    public DateThread(SimpleDateFormat sdf, String dateString) {
        this.sdf = sdf;
        this.dateString = dateString;
    }
    public static SimpleDateFormat getSimpleDateFormat(String datePattern) {
        SimpleDateFormat sdf = null;
        sdf = tl.get();
        if (sdf == null) {
            sdf = new SimpleDateFormat(datePattern);
            tl.set(sdf);
        }
        return sdf;
    }
    @Override
    public void run() {
        try {

            Date date = getSimpleDateFormat("yyyy-MM-dd").parse(dateString);
            String newDateString = DateTools2.getSimpleDateFormat("yyyy-MM-dd").format(date).toString();
            System.out.println(newDateString);
            if (!newDateString.equals(dateString)) {
                System.out.println("ThreadName = " + this.getName()
                        + "報錯了 日期字串：" + dateString
                        + " 轉換成的日期為" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}
public class Run3_formatError {
    public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        String[] dateStringArray = new String[] {
                "2018-12-01", "2018-12-02", "2018-12-03", "2018-12-04", "2018-12-05",
                "2018-12-06", "2018-12-07", "2018-12-08", "2018-12-09", "2018-12-10"
        };
        DateThread[] threads = new DateThread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new DateThread(sdf, dateStringArray[i]);
        }

        for (int i = 0; i < 10; i++) {
            threads[i].start();
        }
    }
}