1.SimpleDateFormat线程不安全
输出时间不安全

private StringBuffer format(Date date, StringBuffer toAppendTo,
                                FieldDelegate delegate) {
        // Convert input date to time field list
        calendar.setTime(date);

        boolean useDateFormatSymbols = useDateFormatSymbols();

        for (int i = 0; i < compiledPattern.length; ) {
            int tag = compiledPattern[i] >>> 8;
            int count = compiledPattern[i++] & 0xff;
            if (count == 255) {
                count = compiledPattern[i++] << 16;
                count |= compiledPattern[i++];
            }

public abstract class DateFormat extends Format {
    protected Calendar calendar;

public class SimpleDateFormat extends DateFormat

可以看出calendar当多个线程访问的时候会存在不安全的情况，因为calendar数值可能被改变。

输出格式线程不安全
再看SimpleDateFormat的构造器

    public SimpleDateFormat(String pattern, Locale locale)
    {
        if (pattern == null || locale == null) {
            throw new NullPointerException();
        }
        initializeCalendar(locale);
        this.pattern = pattern;
        this.formatData = DateFormatSymbols.getInstanceRef(locale);
        this.locale = locale;
        initialize(locale);
    }

追踪pattern发现pattern数值可被修改

 public void applyLocalizedPattern(String pattern) {
         String p = translatePattern(pattern,
                                     formatData.getLocalPatternChars(),
                                     DateFormatSymbols.patternChars);
         compiledPattern = compile(p);
         this.pattern = p;
    }

测试样例：

    public static void main(String[] args){
        SimpleDateFormat df=new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        Date date=new Date();
        System.out.println(df.format(date));
        df.applyLocalizedPattern("G");
        System.out.println(df.format(date));
    }

测试结果：

2022-07-05 09:35:30
公元

确保时间安全的样例代码：

    public static final SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");

    public static void main(String[] args) {
        synchronized (df){
            Date date = new Date();
            System.out.println(df.format(date));
        }
    }

但是final修饰仍然可以改变pattern 。

2.DateTimeFormatter线程安全

所有变量包括他自己都是使用final修饰的。

public final class DateTimeFormatter {

    /**
     * The printer and/or parser to use, not null.
     */
    private final CompositePrinterParser printerParser;
    /**
     * The locale to use for formatting, not null.
     */
    private final Locale locale;
    /**
     * The symbols to use for formatting, not null.
     */
    private final DecimalStyle decimalStyle;
    /**
     * The resolver style to use, not null.
     */
    private final ResolverStyle resolverStyle;
    /**
     * The fields to use in resolving, null for all fields.
     */
    private final Set<TemporalField> resolverFields;
    /**
     * The chronology to use for formatting, null for no override.
     */
    private final Chronology chrono;
    /**
     * The zone to use for formatting, null for no override.
     */
    private final ZoneId zone;

使用样例：

		DateTimeFormatter dateTimeFormatter=DateTimeFormatter.ofPattern("yyyy-MM-dd hh:mm:ss");
        String str1 = dateTimeFormatter.format(LocalDateTime.now());
        System.out.println(str1);
        String dateStr= "2018年12月18日";
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy年MM月dd日");
        LocalDate date2= LocalDate.parse(dateStr, formatter);
        dateTimeFormatter.parse(str1);

根据查看源码，不涉及对自身属性或者方法的修改，可以认为只是方法调用，

    public void formatTo(TemporalAccessor temporal, Appendable appendable) {
        Objects.requireNonNull(temporal, "temporal");
        Objects.requireNonNull(appendable, "appendable");
        try {
            DateTimePrintContext context = new DateTimePrintContext(temporal, this);
            if (appendable instanceof StringBuilder) {
                printerParser.format(context, (StringBuilder) appendable);
            } else {
                // buffer output to avoid writing to appendable in case of error
                StringBuilder buf = new StringBuilder(32);
                printerParser.format(context, buf);
                appendable.append(buf);
            }
        } catch (IOException ex) {
            throw new DateTimeException(ex.getMessage(), ex);
        }
    }

printerParser成员是final修饰，printerParser.format方法代码如下，其中成员属性optional也是final修饰，

 public boolean format(DateTimePrintContext context, StringBuilder buf) {
            int length = buf.length();
            if (optional) {
                context.startOptional();
            }
            try {
                for (DateTimePrinterParser pp : printerParsers) {
                    if (pp.format(context, buf) == false) {
                        buf.setLength(length);  // reset buffer
                        return true;
                    }
                }
            } finally {
                if (optional) {
                    context.endOptional();
                }
            }
            return true;
        }

通过查询发现parse也是如此。