Java 基础知识汇总 - 03

原创 于 2015-01-16 09:53:38 发布 · 483 阅读 · 0 ·
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文章标签：

#多线程 #java #并发 #thread #线程

本文详细解析了Java中的多线程同步机制，包括synchronized关键字的应用及其对资源加锁的作用，以及如何避免线程冲突。此外，文章还深入探讨了Java内部类的基本用法，并对比了静态内部类的特性。

1.java中的字符串的长度和字节数

java中汉字占两个字节占2个字节，但字符串长度是1；英文字符（如A），占1个字节，长度是1,下面的例子能够说明.

2.java synchronized关键字

多线程的同步机制对资源进行加锁，使得在同一个时间，只有一个线程可以进行操作，同步用以解决多个线程同时访问时可能出现的问题，java是通过synchronized关键字来实现的。

当synchronized关键字修饰一个方法的时候，该方法叫做线程同步方法，简称同步方法；

当synchronized方法执行完或发生异常时，会自动释放锁.

下面的例子里面的Printer类有一个print()的方法，该方法是synchronized的，所以同时只能由一个线程去执行，所以，下面的结果应该是一个线程先输出0-9，然后再输出另一个线程的0-9.代码如下：

public class Hello {
	
	public static void main(String[] args) {
		Printer p = new Printer();

        Thread t1 = new MyThread(p);
        Thread t2 = new MyThread(p);

        t1.start();
        t2.start();
	}
}

class Printer
{
    public synchronized void Print()
    {
        for (int i = 0; i < 10; ++i)
        {
            try
            {
                Thread.sleep(500);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +" executing,print message:Hello: " + i);
        }
    }
}

class MyThread extends Thread
{
    private Printer p;

    public MyThread(Printer p)
    {
        this.p = p;
    }

    @Override
    public void run()
    {
    	
        p.Print();
    }
}

输出结果：

Thread-0 executing,print message:Hello: 0
Thread-0 executing,print message:Hello: 1
Thread-0 executing,print message:Hello: 2
Thread-0 executing,print message:Hello: 3
Thread-0 executing,print message:Hello: 4
Thread-0 executing,print message:Hello: 5
Thread-0 executing,print message:Hello: 6
Thread-0 executing,print message:Hello: 7
Thread-0 executing,print message:Hello: 8
Thread-0 executing,print message:Hello: 9
Thread-1 executing,print message:Hello: 0
Thread-1 executing,print message:Hello: 1
Thread-1 executing,print message:Hello: 2
Thread-1 executing,print message:Hello: 3
Thread-1 executing,print message:Hello: 4
Thread-1 executing,print message:Hello: 5
Thread-1 executing,print message:Hello: 6
Thread-1 executing,print message:Hello: 7
Thread-1 executing,print message:Hello: 8
Thread-1 executing,print message:Hello: 9

可以很清楚地看到，第一个线程先输出，等第一个线程先输出完，然后再输出第二个线程。那么如果不加synchronized关键字会怎么样呢？

public void Print()
    {
        for (int i = 0; i < 10; ++i)
        {
            try
            {
                Thread.sleep(500);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +" executing,print message:Hello: " + i);
        }
    }

输出结果如下：

Thread-1 executing,print message:Hello: 0
Thread-0 executing,print message:Hello: 0
Thread-1 executing,print message:Hello: 1
Thread-0 executing,print message:Hello: 1
Thread-0 executing,print message:Hello: 2
Thread-1 executing,print message:Hello: 2
Thread-0 executing,print message:Hello: 3
Thread-1 executing,print message:Hello: 3
Thread-1 executing,print message:Hello: 4
Thread-0 executing,print message:Hello: 4
Thread-1 executing,print message:Hello: 5
Thread-0 executing,print message:Hello: 5
Thread-0 executing,print message:Hello: 6
Thread-1 executing,print message:Hello: 6
Thread-0 executing,print message:Hello: 7
Thread-1 executing,print message:Hello: 7
Thread-0 executing,print message:Hello: 8
Thread-1 executing,print message:Hello: 8
Thread-0 executing,print message:Hello: 9
Thread-1 executing,print message:Hello: 9

可以看到两个线程在穿插进行地执行，每次输出的结果都不完全相同，这个取决于CPU对线程的调度情况。

另外，还有一种对执行的方法加锁的方式，具体如下：

public void Print(){
    	synchronized(this){
    	for (int i = 0; i < 10; ++i)
        {
            try
            {
                Thread.sleep(500);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() +" executing,print message:Hello: " + i);
        }
    }
}

这样输出的结果和在方法上使用synchronized关键字进行修饰是一样的，不再赘述。

这里需要注意的是：当一个线程访问object的一个synchronized(this)同步代码块时，另一个线程仍然可以访问该object中的非synchronized(this)同步代码块！！！

public class Hello {
	
	public static void main(String[] args) {
		Printer p = new Printer();

        Thread t1 = new MyThread(p);
        Thread t2 = new MyThread(p);

        t1.start();
        t2.start();
	}
}

class Printer
{
    public void Print()
    {
    	synchronized(this){
    		for (int i = 0; i < 10; ++i)
            {
                try
                {
                    Thread.sleep(500);
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() +" executing sync,print message:Hello: " + i);
            }
    	}
    	
    	for(int i = 0; i< 10;i++){
    		try {
				Thread.sleep(500);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
    		System.out.println(Thread.currentThread().getName() +" executing none sync,print message:World: " + i);
    	}
    }
}

class MyThread extends Thread
{
    private Printer p;

    public MyThread(Printer p)
    {
        this.p = p;
    }

    @Override
    public void run()
    {
    	
        p.Print();
    }
}

输出结果：

Thread-0 executing sync,print message:Hello: 0
Thread-0 executing sync,print message:Hello: 1
Thread-0 executing sync,print message:Hello: 2
Thread-0 executing sync,print message:Hello: 3
Thread-0 executing sync,print message:Hello: 4
Thread-0 executing sync,print message:Hello: 5
Thread-0 executing sync,print message:Hello: 6
Thread-0 executing sync,print message:Hello: 7
Thread-0 executing sync,print message:Hello: 8
Thread-0 executing sync,print message:Hello: 9
Thread-1 executing sync,print message:Hello: 0
Thread-0 executing none sync,print message:World: 0
Thread-1 executing sync,print message:Hello: 1
Thread-0 executing none sync,print message:World: 1
Thread-0 executing none sync,print message:World: 2
Thread-1 executing sync,print message:Hello: 2
Thread-0 executing none sync,print message:World: 3
Thread-1 executing sync,print message:Hello: 3
Thread-0 executing none sync,print message:World: 4
Thread-1 executing sync,print message:Hello: 4
Thread-1 executing sync,print message:Hello: 5
Thread-0 executing none sync,print message:World: 5
Thread-1 executing sync,print message:Hello: 6
Thread-0 executing none sync,print message:World: 6
Thread-1 executing sync,print message:Hello: 7
Thread-0 executing none sync,print message:World: 7
Thread-1 executing sync,print message:Hello: 8
Thread-0 executing none sync,print message:World: 8
Thread-1 executing sync,print message:Hello: 9
Thread-0 executing none sync,print message:World: 9
Thread-1 executing none sync,print message:World: 0
Thread-1 executing none sync,print message:World: 1
Thread-1 executing none sync,print message:World: 2
Thread-1 executing none sync,print message:World: 3
Thread-1 executing none sync,print message:World: 4
Thread-1 executing none sync,print message:World: 5
Thread-1 executing none sync,print message:World: 6
Thread-1 executing none sync,print message:World: 7
Thread-1 executing none sync,print message:World: 8
Thread-1 executing none sync,print message:World: 9

这个结果有点冗长，仔细观察发现sync方法是线程0先执行，执行完了线程2才开始执行，而none sync方法穿插着执行，这个印证了上述结论。结论就是：

synchronized方法是一种粗粒度的并发控制，某一时刻，只能有一个线程执行该synchronized方法；
synchronized块则是一种细粒度的并发控制，只会将块中的代码同步，位于方法内、synchronized块之外的其他代码是可以被多个线程同时访问到的。
3.内部类

1）基本用法

public class HelloWorld {
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		OutClass.InClass ic = new OutClass().new InClass();
		ic.print();
	}
}

class OutClass {
    private int age = 12;
    class InClass {
        public void print() {
            System.out.println(age);
        }
    }
}

还有一种内部类是静态内部类，只能访问外部类的静态变量！（注释里面的访问方式是不允许的！）

class Out {
    private static int age = 12;
    private int salary;
    static class In {
        public void print() {
            System.out.println(age);
            //System.out.println(salary);
        }
    }
}



 


 



 


 

 

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