java-tutorial-learn-examples

my codes when learn java core .

项目结构

• src 目录
  以包区分不同的学习知识点.
  

包目录

    ├── org.java.tutorial             // 根目录
          ├── nestedClass           // 内部类
          ├── exceptions        // java 核心类Exception
          ├── basicIo          // 文件读取io处理
               ├── fileIo
               ├── ioStreams
          ├── concurrency      //并发
               ├── threadObjects   //thread对象的创建和基本静态方法
               ├── synchronization  //同步处理的方法和策略
               ├── liveness     //处理更复杂的线程饥饿,死锁等问题

##学习方法 根据包一个类一个知识点的学习,跑测试用例.

learn Notes

BasicIo

• 大多数关于I/O Streams部分的类都在java.io 包中.大多数关于File I/O的部分的类都在java.nio.file包中.*

• I/O Streams a stream is a sequence of data. a program use an input stream to read data from a source ,one item at a time. a program uses a output stream write data to a destination,ont item at a time.

• Byte streams Programs use byte streams to perform input and output of 8-bit bytes. All byte stream classes are descended from InputStream and OutputStream. 所有的stream 类信息都是建立在byte stream 之上的. 根据stream要传递的资源类型选择不同的stream处理,可以提高效率.

• Character streams The Java platform stores character values using Unicode conventions. Character stream I/O automatically translates this internal format to and from the local character set. In Western locales, the local character set is usually an 8-bit superset of ASCII. For most applications, I/O with character streams is no more complicated than I/O with byte streams. Input and output done with stream classes automatically translates to and from the local character set. A program that uses character streams in place of byte streams automatically adapts to the local character set and is ready for internationalization — all without extra effort by the programmer.

一行的分隔符可以是("\r\n"), 回车("\r"), 换行符("\n"). 不同系统分隔符可能不一样.

• Buffered Streams 为了减轻直接os操作引起的效率问题,java实现了i/o buffered streams.buffer i/o从内存读取数据,只有当buffer空了的时候才会调用直接os操作. 可以将unbuffered streams 转换为buffered streams.

inputStream = new BufferedReader(new FileReader("xanadu.txt"));
outputStream = new BufferedWriter(new FileWriter("characteroutput.txt"));

There are four buffered stream classes used to wrap unbuffered streams: BufferedInputStream and BufferedOutputStream create buffered byte streams, while BufferedReader and BufferedWriter create buffered character streams.

• Flushing Buffered Streams It often makes sense to write out a buffer at critical points, without waiting for it to fill. This is known as flushing the buffer.

• Scanning Objects of type Scanner are useful for breaking down formatted input into tokens and translating individual tokens according to their data type.

• random access files 使用SeekableByteChannel接口定义可以实现这个功能.因为它继承的channel I/O有记录当前位置的标记.这里几个方法:

  position – Returns the channel's current position position(long) – Sets the channel's position read(ByteBuffer) – Reads bytes into the buffer from the channel write(ByteBuffer) – Writes bytes from the buffer to the channel truncate(long) – Truncates the file (or other entity) connected to the channel

• 方法介绍:Buffer java.nio.Buffer.rewind(). Rewinds this buffer. The position is set to zero and the mark is discarded.

Invoke this method before a sequence of channel-write or get operations, assuming that the limit has already been set appropriately. For example:

out.write(buf); // Write remaining data buf.rewind(); // Rewind buffer buf.get(array); // Copy data into array

• walking a file tree 首先要实现接口FileVistor: preVisitDirectory – Invoked before a directory's entries are visited. postVisitDirectory – Invoked after all the entries in a directory are visited. If any errors are encountered, the specific exception is passed to the method. visitFile – Invoked on the file being visited. The file's BasicFileAttributes is passed to the method, or you can use the file attributes package to read a specific set of attributes. For example, you can choose to read the file's DosFileAttributeView to determine if the file has the "hidden" bit set. visitFileFailed – Invoked when the file cannot be accessed. The specific exception is passed to the method. You can choose whether to throw the exception, print it to the console or a log file, and so on. 如果你不需要实现FileVistor接口中所有的方法,那么你可以继承SimpleFileVisitor,然后选择override你所需要的方法.

Concurrency

• processes and threads(进程和线程) *

线程也称为轻量级的进程.它们都有自己的执行环境.但是创建一个线程比创建一个进程需要 更少的资源..线程在进程中,并且一个进程包含至少一个线程.线程分享进程的资源.java应用启动会创建一个主线程,称为main thread.main thread 可以创建多个线程.

Thread Objects

每一个线程都是一个Thread对象实例.有2这种方式创建并行应用. 1.每当需要执行异步任务时,直接初始化一个Thread对象. 2.使用抽象的thread管理应用,在需要异步任务时,传递任务给一个executor.

定义和启动一个线程

创建Thread实例必须定义在线程中执行的代码,有2种方式: 1.提供一个Runnable对象.Runnable接口定义了一个唯一的run方法,意味着在run方法中写线程执行的代码.Runnable对象会被传递给Thread构造函数. 2.继承Thread.覆盖Thread对象的run方法.

• 一般我们选择使用第一种方法创建一个Thread对象,因为第一种方法中,Runnable对象可以继承自己定义的线程类而第二种只能是Thread子类. *

使用Sleep方法暂停当前线程的执行

Thread.sleep()让当前线程暂停一段时间执行.

Interrupts

中断意味着,这个线程需要停止当前执行的任务,去做其他的任务. Interrupt 机制的实现通过一个interrupt status的标识.调用Thread.interrupt会设置这个标识.当一个线程调用Thread.interrupted(),interrupt status标识会清除.非静态方法isInterrupted():通过调用一个线程去查询另外一个线程的状态,则不会改变interrupt status标识.抛出InterruptedException异常时,interrupt status标识也会清除.

• 非静态方法public void interrupt()会设置interrupt status标识,并且让当前线程中断该线程.
• 静态方法public static boolean interrupted()检测是否当前线程已经中断了,interrupt status标识会清除.
• 非静态方法public boolean isInterrupted()检测该线程是否已经中断了,通过调用一个线程去查询另外一个线程的状态,不会改变interrupt status标识.

join

join()方法允许一个线程等待另外一个线程的的完成.假如一个线程t.join().会导致当前线程暂停,执行t线程,直到t线程执行结束. 当A线程执行到了B线程的join()方法时，A就会等待，等B线程都运行完，A线程才会执行。使用join()方法时，会产生异常。 join方法可以用于临时加入线程，一个线程在运算过程中，如果满足于条件，我们可以临时加入一个线程，让这个线程运算完，另外一个线程再继续运行。

Synchronization

Thread Interference(线程干扰)

使用synchronized关键字

标注synchronized关键字的方法可以保证同步,一个时间只能有一个线程访问,其余线程只能阻塞.

• 构造方法不能使用synchronized 关键字.* 1.使用Synchronized关键字 2.使用synchronized statement. 3.使用锁. 4.Reentrant Synchronization:一个线程不能获取到另外一个线程控制的锁.但是一个线程可以获取到它本身已经拥有的锁. 允许一个线程多次获取同一个锁就是 reentrant synchronization

atomic access

要么全部执行完成,要么完成不执行.类似于原子操作. 原子操作的有: 1.引用变量,基本变量(除了long和double)的读和写. 2.申明为volatile 的变量.

liveness