《Java多线程编程核心技术》读后感（十七）

使线程具有有序性

正常情况下，线程在运行时多个线程之间执行任务的时机时无序的。可以通过改造代码的方式使它们运行具有有序性

package Seven;
 
public class MyThread extends Thread {
 
    private Object lock;
    private String showChar;
    private int showNumPosition;
 
    private int printCount = 0;// 统计打印了几个字母
 
    volatile private static int addNumber = 1;
 
    public MyThread(Object lock, String showChar, int showNumPosition) {
        super();
        this.lock = lock;
        this.showChar = showChar;
        this.showNumPosition = showNumPosition;
    }
 
    @Override
    public void run() {
        try {
            synchronized (lock) {
                while (true) {
                    if (addNumber % 3 == showNumPosition) {
                        System.out.println("ThreadName="
                                + Thread.currentThread().getName()
                                + " runCount=" + addNumber + " " + showChar);
                        lock.notifyAll();
                        addNumber++;
                        printCount++;
                        if (printCount == 3) {
                            break;
                        }
                    } else {
                        lock.wait();
                    }
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
 
    }
 
}

package Seven;
 
public class Run {
    public static void main(String[] args) {
 
        Object lock = new Object();
 
        MyThread a = new MyThread(lock, "A", 1);
        MyThread b = new MyThread(lock, "B", 2);
        MyThread c = new MyThread(lock, "C", 0);
 
        a.start();
        b.start();
        c.start();
 
    }
}

SimpleDateFormat非线程安全

1在多线程环境下出现异常

package Seven;
 
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
 
public class MyThread extends Thread {
 
    private SimpleDateFormat sdf;
    private String dateString;
 
    public MyThread(SimpleDateFormat sdf, String dateString) {
        super();
        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=" + this.getName()+ "报错了 日期字符串：" + dateString + " 转换成的日期为："+ newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
 
    }
 
}

package Seven;
 
import java.text.SimpleDateFormat;
 
public class Test {
    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();
        }
 
    }
}

2、解决异常方法1

package Seven;
 
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
 
public class MyThread extends Thread {
 
    private SimpleDateFormat sdf;
    private String dateString;
 
    public MyThread(SimpleDateFormat sdf, String dateString) {
        super();
        this.sdf = sdf;
        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=" + this.getName()    + "报错了 日期字符串：" + dateString + " 转换成的日期为："+ newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
 
    }
 
}

package Seven;
 
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
 
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) {
        return new SimpleDateFormat(formatPattern).format(date).toString();
    }
 
}

package Seven;
 
import java.text.SimpleDateFormat;
 
public class Test {
    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();
        }
 
    }
}

解决的原理是创建了多个SimpleDateFormat类的实例

解决异常方法2

package Seven;
 
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
 
public class MyThread extends Thread {
 
    private SimpleDateFormat sdf;
    private String dateString;
 
    public MyThread(SimpleDateFormat sdf, String dateString) {
        super();
        this.sdf = sdf;
        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=" + this.getName()+ "报错了 日期字符串：" + dateString + " 转换成的日期为："+ newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
 
    }
 
}

package Seven;
 
import java.text.SimpleDateFormat;
 
public class DateTools {
    private static ThreadLocal<SimpleDateFormat> tl = new ThreadLocal<SimpleDateFormat>();
 
    public static SimpleDateFormat getSimpleDateFormat(String datePattern) {
        SimpleDateFormat sdf = null;
        sdf = tl.get();
        if (sdf == null) {
            sdf = new SimpleDateFormat(datePattern);
            tl.set(sdf);
        }
        return sdf;
    }
}

package Seven;
 
import java.text.SimpleDateFormat;
 
public class Test {
    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();
        }
 
    }
}

原理是ThreadLocal类能使线程绑定到指定的对象