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          java作为跨平台的 静态类型  动态分派的语言, 还有垃圾收集器, 天生就比C++效率差一点. 但是随着java的发展, 对于java性能的优化可以说达到极致了.  java语言的效率可以说丝毫不差于C++了
编译期分为  早期编译期 和即时编译期####早期编译期: 将java代码编辑成 class字节码
这个编辑成字节码的工具有很多, 这里主要讲javac 在编译时候所做的对于java运行性能的优化
javac也是有java语言实现的, 所以可以学习其源码
编译过程分为:

解析与填充符号表的过程

词法语法分析,   就是将字符流转换成对应的数据结果,  就是解析字符串

插入式注解
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          概述java类编译成class后我们就没有办法再来影响jvm虚拟机了,    所以java中的大部分看似很灵活的特性编译成字节码后都是最基本的特性, 比如泛型就是Object,  枚举类就是final类,  常量就会替换成具体值, 注解就是一个接口.  等等这些特性都是通过改变字节码的生成来实现的,  也有很多的字节码操作库来控制生成字节码 来达到很多不可思议的功能,  比如Groovy的动态语言特性 ,   也可以通过自定义类加载器来控制类的行为
tomcat正统的类加载器结构主流的web服务器都实现类自己的类加载器, 这是因为web服务需要解决如下的功能:

部署在同一个服务器上的两个服
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          虚拟机字节码执行引擎概述物理机的执行引擎是直接建立在处理器、硬件、指令集和操作系统层面上的，而虚拟机的执行引擎则是由自己实现的，因此可以自行制定指令集与执行引擎的结构体系，并且能够执行那些不被硬件直接支持的指令集格式。
运行时栈帧结构栈帧存储了方法的局部变量表、操作数栈、动态连接和方法返回地址等信息。每一个方法从调用开始至执行完成的过程，都对应着一个栈帧在虚拟机栈里面从入栈到出栈的过程。
每一个栈帧都包括了局部变量表、操作数栈、动态连接、方法返回地址和一些额外的附加信息。
对于执行引擎来说，在活动线程中，只有位于栈顶的栈帧才是有效的，称为当前栈帧。

局部变量表
在方法体内部定义的变量都是 在
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          1.概述Class文件中包含了类定义的各种信息, 这些信息都必须加载到jvm中才会有用的. 虚拟机吧描述类的数据从class文件加载到内存中, 并对数据进行校验, 转换解析和初始化, 最终形成可以被虚拟机直接使用的java类型就是 类加载机制

上面只是官方的描述, 简单说就是包class文件加载到内存中后可以被程序使用的过程

java的类加载机制与c++/c 有所不同, c++和c在程序编译成exe后就已经与操作系统绑定了, 将程序的所有可能都遍历保存在exe中了,  而Java程序是在程序启动后动态的加载class文件的. 就是在程序运行期间完成,  这样虽然增加了程序运行的开销,但是大
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          深入理解java虚拟机
jvm的内存区域， Hotspot在jdk7中将方法区和字符串常量池移动到堆中了
jdk8移除了方法去 使用元空间替代，元空间也使用堆内存
1， 程序计数器，每个线程私有记录当前线程所执行指令的行号， 根据这个行号方便程序进行调整， 比如循环，分支，跳转， 异常跳转等
2. 虚拟机栈   每个线程独有每个方法在执行的时候都会产生一个栈帧， 方法里面的局部变量（值或者应用） 就放入栈帧中。 执行完了这个栈帧就退出栈了， 所以不需要额外垃圾回收机制， 但是如果太深的方法调用（递归）就可能耗尽栈内存，导致栈溢出异常
\3. 本地方法栈，  与虚拟机栈类似， jvm规范中并没有
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          内存屏障（Memory barrier）为什么会有内存屏障
每个CPU都会有自己的缓存（有的甚至L1,L2,L3），缓存的目的就是为了提高性能，避免每次都要向内存取。但是这样的弊端也很明显：不能实时的和内存发生信息交换，分在不同CPU执行的不同线程对同一个变量的缓存值不同。
用volatile关键字修饰变量可以解决上述问题，那么volatile是如何做到这一点的呢？那就是内存屏障，内存屏障是硬件层的概念，不同的硬件平台实现内存屏障的手段并不是一样，java通过屏蔽这些差异，统一由jvm来生成内存屏障的指令。


内存屏障是什么
硬件层的内存屏障分为两种：Load Barrier 和 Store
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          同步  异步,  阻塞 非阻塞.
可以类比点外卖的情况.
同步就是需要你自己亲自去取外卖, 异步就是有人给你送到家
阻塞和 非阻塞就是是否需要在店铺排队等待 取餐
同步阻塞 就是你自己亲自到店铺排队取餐
同步非阻塞  就是你定个餐,   店铺做好还打电话通知你, 你自己去取
异步阻塞 就是叫个快递员去排队取餐,  快递员的时间也是很宝贵的, 这个方式明显不划算,
异步非阻塞   你订餐后, 店铺做好后直接让快递员送给你,
异步方式 始终有一个快递员的 花费,  有点代价,  著名的netty框架抛弃AIO 专注使用NIO 还是有一定道理的
###BIO是java1.4之前就有的,
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写在前面
闭锁
信号量
栅栏
阻塞队列


CAS还是借助native的方法实现.  CAS是乐观锁, 比较底层, 性能较高, 但是CAS中有大量的自旋操作, 又会消耗性能, 世上没有完美的解决方案

CountDownLatch 与 CyclicBarrier 区别.
CountDownLatch 可以多个线程都持有这个对象后调用await方法等待,
当计数器为0的时候, 就会唤醒所有的等待的线程


CyclicBarrier  没有计数器(显示的), 只有规定个数的线程都到了await状态后, 才会把这规定个数的线程一起唤醒继续执行.  感觉就像 所有的选手都到了统一的位置后再次开
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TryLock 就是尝试一下获取锁,  尝试完后马上返回结果(不阻塞), 如果获取到了锁就返回true, 没有就返回false

lock 与 lockInterruptibly比较区别在于：lock 优先考虑获取锁，待获取锁成功后，才响应中断。lockInterruptibly 优先考虑响应中断，而不是响应锁的普通获取或重入获取。

区别一个在获取的过程中可以中断, 另一个在获取的过程中无法中断, 非要等到获取后才能中断

详细区别：ReentrantLock.lockInterruptibly允许在等待时由其它线程调用等待线程的Thread.interrupt方法来中断等待线程的等待而直
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 stop方法停止线程是不安全的方法,  是通过异常来停止线程的, 线程捕捉到异常后会释放所有的锁后停止线程,  释放所有的锁就会有问题,
interrupt 方式停止线程实际上是 通过线程停止标识来停止线程的,  线程在sleep和wait等阻塞函数的时候会一直 不断的轮询标识, 如果标识改变了就会终止线程.  至于他抛出的异常是主动抛出的, 并且需要显式的捕捉处理的

1、为什么废弃Thread的stop函数？对于有多线程开发经验的开发者，应该大多数在开发过程中都遇到过这样的需求，就是在某种情况下，希望立即停止一个线程。
比如：做Android APP开发，当打开一个界面时，需要开
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      var url = $visitors.attr('id').trim();
      var title = $visitors.attr('data-flag-title').trim();
      var query = new AV.Query(Counter);

      query.equalTo("url", url);
      query.find({
        success: function(results) {
          if (results.length > 0) {
            var counter = results[0];
            counter.fetchWhenSave(true);
            counter.increment("time");
            counter.save(null, {
              success: function(counter) {
                var $element = $(document.getElementById(url));
                $element.find('.leancloud-visitors-count').text(counter.get('time'));
              },
              error: function(counter, error) {
                console.log('Failed to save Visitor num, with error message: ' + error.message);
              }
            });
          } else {
            var newcounter = new Counter();
            /* Set ACL */
            var acl = new AV.ACL();
            acl.setPublicReadAccess(true);
            acl.setPublicWriteAccess(true);
            newcounter.setACL(acl);
            /* End Set ACL */
            newcounter.set("title", title);
            newcounter.set("url", url);
            newcounter.set("time", 1);
            newcounter.save(null, {
              success: function(newcounter) {
                var $element = $(document.getElementById(url));
                $element.find('.leancloud-visitors-count').text(newcounter.get('time'));
              },
              error: function(newcounter, error) {
                console.log('Failed to create');
              }
            });
          }
        },
        error: function(error) {
          console.log('Error:' + error.code + " " + error.message);
        }
      });
    }

    $(function() {
      var Counter = AV.Object.extend("Counter");
      if ($('.leancloud_visitors').length == 1) {
        addCount(Counter);
      } else if ($('.post-title-link').length > 1) {
        showTime(Counter);
      }
    });
  </script>


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