/**

• 線程類
  */
  public class MyThread extends Thread {

  private SimpleDateFormat sdf;
  private String dateString;

  public MyThread(SimpleDateFormat sdf,String dateString) {
  

  this.sdf = sdf;
  this.dateString = dateString;
  }

  @Override
  public void run() {
  try {
  Date dateRef = sdf.parse(dateString);
  String newDateString = sdf.format(dateRef).toString();
  if(!newDateString.equals(dateString)) {
  System.out.println("ThreadName = " + Thread.currentThread().getName()

  • "報錯了 日期字符串：" + dateString + "轉換成日期為：" + newDateString);
    
    }
    } catch (ParseException e) {
    e.printStackTrace();
    }
    }
    }
    import java.text.SimpleDateFormat;

public class Test {

/**
 *    測試單例的SimpleDateFormat類在多線程環境下中處理日期，極易出現日期轉換錯誤的情況
 */
public static void main(String[] args) {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    String[] dateStringArray = new String[] {
            "2000-01-01","2000-01-02","2000-01-03",
            "2000-01-04","2000-01-05","2000-01-06",
            "2000-01-07","2000-01-08","2000-01-09",
            "2000-01-10"
    };

    MyThread[] threadArray = new MyThread[10];
    for (int i = 0; i < 10; i++) {
        threadArray[i] = new MyThread(sdf, dateStringArray[i]);
    }
    for (int i = 0; i < 10; i++) {
        threadArray[i].start();
    }
}
 

}
運行之後會輸出很多的錯誤信息！
解決多線程出現的問題-為每個線程實例一個SimpleDateFormat：
import java.text.ParseException;
import java.util.Date;

/**

• 線程類
  */
  public class MyThread extends Thread {

  private String dateString;
  public MyThread(String dateString) {
  this.dateString = dateString;
  }

  @Override
  public void run() {
  try {
  Date dateRef = DateTools.parse("yyyy-MM-dd", dateString);
  String newDateString = DateTools.format("yyyy-MM-dd",dateRef).toString();
  if(!newDateString.equals(dateString)) {
  System.out.println("ThreadName = " + Thread.currentThread().getName()

  • "報錯了 日期字符串：" + dateString + "轉換成日期為：" + newDateString);
    }
    } catch (ParseException e) {
    e.printStackTrace();
    }
    }
    }
    import java.text.ParseException;
    import java.text.SimpleDateFormat;
    import java.util.Date;

/**

• 日期格式化工具類
  */
  public class DateTools {

  public static Date parse(String formatPattern,String dateString) throws ParseException {
  return new SimpleDateFormat(formatPattern).parse(dateString);
  }

  public static String format(String formatPattern,Date date) throws ParseException {
  return new SimpleDateFormat(formatPattern).format(date).toString();
  }
  }
  public class Test {

  /**

  • 測試,運行程序後，控制臺沒有任何輸出，也就是轉換沒有任何異常，

  • 原理：創建了多個SimpleDateFormat實例
    */
    public static void main(String[] args) {
    String[] dateStringArray = new String[] {
    "2000-01-01","2000-01-02","2000-01-03",
    "2000-01-04","2000-01-05","2000-01-06",
    "2000-01-07","2000-01-08","2000-01-09",
    "2000-01-10"
    };

    MyThread[] threadArray = new MyThread[10];
    for (int i = 0; i < 10; i++) {
    threadArray[i] = new MyThread(dateStringArray[i]);
    }
    for (int i = 0; i < 10; i++) {
    threadArray[i].start();
    }
    }
    }
    解決多線程出現的問題-使用ThreadLocal：
    import java.text.ParseException;
    import java.util.Date;

/**

• 線程類
  */
  public class MyThread extends Thread {

  private String dateString;
  public MyThread(String dateString) {
  this.dateString = dateString;
  }

  @Override
  public void run() {
  try {
  Date dateRef = DateTools.getSimpleDateFormat("yyyy-MM-dd").parse(dateString);
  String newDateString = DateTools.getSimpleDateFormat("yyyy-MM-dd").format(dateRef).toString();
  if(!newDateString.equals(dateString)) {
  System.out.println("ThreadName = " + Thread.currentThread().getName()

  • "報錯了 日期字符串：" + dateString + "轉換成日期為：" + newDateString);
    }
    } catch (ParseException e) {
    e.printStackTrace();
    }
    }
    }
    import java.text.SimpleDateFormat;

/**

• 日期格式化工具類,使用ThreadLocal解決SimpleDateFormat非線程安全問題
  */
  public class DateTools {

  private static ThreadLocal<SimpleDateFormat> t1 = new ThreadLocal<>();

  public static SimpleDateFormat getSimpleDateFormat(String datePattern) {
  SimpleDateFormat sdf = null;
  sdf = t1.get();
  if(sdf == null) {
  sdf = new SimpleDateFormat(datePattern);
  t1.set(sdf);
  }
  return sdf;
  }
  }
  public class Test {

  /**

  • 測試,運行程序後，控制臺沒有任何輸出，也就是轉換沒有任何異常

  • 原理：每個線程都會有自己的ThreadLocal存儲全局變量，也就是每個線程都有自己的SimpleDateFormat實例
    */
    public static void main(String[] args) {
    String[] dateStringArray = new String[] {
    "2000-01-01","2000-01-02","2000-01-03",
    "2000-01-04","2000-01-05","2000-01-06",
    "2000-01-07","2000-01-08","2000-01-09",
    "2000-01-10"
    };

    MyThread[] threadArray = new MyThread[10];
    for (int i = 0; i < 10; i++) {
    threadArray[i] = new MyThread(dateStringArray[i]);
    }
    for (int i = 0; i < 10; i++) {
    threadArray[i].start();
    }
    }
    }

小編帶你了解SimpleDateFormat-多線程問題