以下程式碼會報錯：

package com.expgiga.Java8;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;

/**
 * 測試傳統時間格式化的執行緒安全問題
*/
public class TestSimpleDateFormat {
    public static void main(String[] args) throws ExecutionException, InterruptedException {

        SimpleDateFormat sdf = new 
 
SimpleDateFormat("yyyyMMdd");

        Callable<Date> task = new Callable<Date>() {
            @Override
public Date call() throws Exception {
                return sdf.parse("20170806");
            }
        };

        ExecutorService pool = Executors.newFixedThreadPool(10);

        List<Future<Date>> results = new 
 
ArrayList<>();

        for (int i = 0; i < 10; i++) {
            results.add(pool.submit(task));
        }

        for (Future<Date> future : results) {
            System.out.println(future.get());
        }
    }
}

Java8之前的解決方法：

package com.expgiga.Java8;

import java.text.DateFormat;
import 
 
java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 *
 */
public class DateFormatThreadLocal {

    private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {
        protected DateFormat initialValue() {
            return new SimpleDateFormat("yyyyMMdd");
        }
    };

    public static Date convert(String source) throws ParseException {
        return df.get().parse(source);
    }
}

package com.expgiga.Java8;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;

/**
 * 測試傳統時間格式化的執行緒安全問題
*/
public class TestSimpleDateFormat {
    public static void main(String[] args) throws ExecutionException, InterruptedException {

//        SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
Callable<Date> task = new Callable<Date>() {
            @Override
public Date call() throws Exception {
//                return sdf.parse("20170806");
return DateFormatThreadLocal.convert("20170805");
            }
        };

        ExecutorService pool = Executors.newFixedThreadPool(10);

        List<Future<Date>> results = new ArrayList<>();

        for (int i = 0; i < 10; i++) {
            results.add(pool.submit(task));
        }

        for (Future<Date> future : results) {
            System.out.println(future.get());
        }

        pool.shutdown();
    }
}

Java8的解決方法：

package com.expgiga.Java8;

import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;

/**
 * 測試傳統時間格式化的執行緒安全問題
*/
public class TestSimpleDateFormat {
    public static void main(String[] args) throws ExecutionException, InterruptedException {


        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyyMMdd");

        Callable<LocalDate> task = new Callable<LocalDate>() {
            @Override
public LocalDate call() throws Exception {
                return LocalDate.parse("20170806", dtf);
            }
        };

        ExecutorService pool = Executors.newFixedThreadPool(10);

        List<Future<LocalDate>> results = new ArrayList<>();

        for (int i = 0; i < 10; i++) {
            results.add(pool.submit(task));
        }

        for (Future<LocalDate> future : results) {
            System.out.println(future.get());
        }

        pool.shutdown();
    }
}