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ClassLoader 加蝲机制

*** — Sat, 08 Sep 2012 09:58:00 GMT

1. Java Class Loading Mechanism

首先当编译一个Java文�g�Ӟ��~�译器就会在生成的字节码中内�|�一个public�Q�static�Q�final的class字段�Q�该字段属于java.lang.Class�c�d��Q�该class字段使用�Ҏ��讉K��Q�所以可以有�Q?/div>

java.lang.Class clazz = MyClass.class

当class被JVM加蝲�Q�就不再加蝲相同的class。class在JVM中通过�Q�ClassLoader�Q�Package�Q�ClassName�Q�来唯一军_��。ClassLoader指定了一个class的scope�Q�这意味着如果两个相同的包下面的class被不同的ClassLoader加蝲�Q�它们是不一��L��Q��ƈ且不是type-compatible的�?/div>

JVM中所有的ClassLoader�Q�bootstrap ClassLoader除外�Q�都是直接或间接�l�承于java.lang.ClassLoader抽象�c�，�q�且��Zؓ逻辑上指定了parent-child关系�Q�实��C��child不一定��承于parent�Q�我们也可以通过�l�承它来实现自己的ClassLoader�?/div>

JVM ClassLoder架构�Q�从上到下依�ơ�ؓparent-child关系�Q?/div>

Bootstrap ClassLoader - 启动�c�d��载器�Q�主要负责加载核心Java�c�d��java.lang.Object和其他运行时所需class�Q�位于JRE/lib目录下或-Xbootclasspath指定的目录。我们不知道�q�多的关于Bootstrap ClassLoader的细节，因�ؓ它是一个native的实玎ͼ�不是Java实现�Q�所以不同JVMs的Bootstrap ClassLoader的行��Z��不尽相同。调�?span style="background-color: #eeeeee; font-size: 13px; ">java.lang.String.getClassLoder() �q�回null�?/span>
sun.misc.ExtClassLoader - 扩展�c�d��载器�Q�负责加载JRE/lib/ext目录�?Djava.ext.dirs指定目录�?/li>
sun.misc.AppClassLoader - 应用�c�d��载器�Q�负责加载java.class.path目录
另外�Q�还有一些其他的ClassLoader如：java.net.URLClassLoader�Q�java.security.SecureClassLoader�Q�java.rmi.server.RMIClassLoader�Q�sun.applet.AppletClassLoader
用户�q�可以自��q��承java.lang.ClassLoader来实现自��q��ClassLoader�Q�用来动态加载class文�g�?/li>

ClassLoader�Ҏ�?/strong>�Q?/div>

每个ClassLoader�l�护一份自��q��命名�I�间�Q�同一个ClassLoader命名�I�间不能加蝲两个同名的类�?/li>
为实现Java安全沙箱模型�Q�默认采用parent-child加蝲铄��构，除Bootstrap ClassLoader没有parent外，每个ClassLoader都有一个逻辑上的parent�Q�就是加载这个ClassLoader的ClassLoader�Q�因为ClassLoader本��n也是一个类�Q�直接或间接的��承java.lang.ClassLoader抽象�c�R�?/li>

java.lang.Thread中包含一个public的方法public ClassLoader getContextClassLoader()�Q�它�q�回某一�U�程相关的ClassLoader�Q�该ClassLoader是线�E�的创徏者提供的用来加蝲�U�程中运行的classes和资源的。如果没有显式的讄��其ClassLoader�Q�默认是parent�U�程的Context ClassLoader。Java默认的线�E�上下文加蝲器是AppClassLoader�?/div>

ClassLoader工作原理�Q?/strong>

了解ClassLoader工作原理�Q�先来看一个ClassLoader�cȝ��化版的loadClass()�Ҏ��源码

1 protected Class loadClass(String name, boolean resolve)
2         throws ClassNotFoundException
3     {
4         synchronized (getClassLoadingLock(name)) {
5             // First, check if the class has already been loaded
6             Class c = findLoadedClass(name);
7             if (c == null) {
8                 long t0 = System.nanoTime();
9                 try {
10                     if (parent != null) {
11                         c = parent.loadClass(name, false);
12                     } else {
13                         c = findBootstrapClassOrNull(name);
14                     }
15                 } catch (ClassNotFoundException e) {
16                     // ClassNotFoundException thrown if class not found
17                     // from the non-null parent class loader
18                 }
19
20                 if (c == null) {
21                     // If still not found, then invoke findClass in order
22                     // to find the class.
24                     c = findClass(name);
25                 }
26             }
27             if (resolve) {
28                 resolveClass(c);
29             }
30             return c;
31         }
32     }

首先查看该class是否已被加蝲�Q�如果已被加载则直接�q�回�Q�否则调用parent的loadClass来加载，如果parent是null代表是Bootstrap ClassLoader�Q�则有Bootstrap ClassLoader来加载，如果都未加蝲成功�Q�最后由该ClassLoader自己加蝲。这�U�parent-child委派模型�Q�保证了恶意的替换Java核心�c�M��会发生，因�ؓ如果定义了一个恶意java.lang.String�Q�它首先会被JVM的Bootstrap ClassLoader加蝲自己JRE/lib下的�Q�而不会加载恶意的。另外，Java允许同一package下的�c�d��以访问受保护成员的访问权限，如定义一个java.lang.Bad�Q�但是因为java.lang.String由Bootstrap ClassLoader加蝲而java.lang.Bad由AppClassLoader加蝲�Q�不是同一ClassLoader加蝲�Q�仍不能讉K��?/div>

2. Hotswap - 热部�|?/strong>

即不重启JVM�Q�直接替换class。因为ClassLoader�Ҏ��，同一个ClassLoader命名�I�间不能加蝲两个同名的类�Q�所以在不重启JVM的情况下�Q�只能通过新的ClassLoader来重新load新的class�?/div>

1  public static void main(String[] args) throws InterruptedException, MalformedURLException {
2         IExample oldExample = new Example();
3         oldExample.plus();
4         System.out.println(oldExample.getCount());
5
6         Hotswap hotswap = new Hotswap();
7         while (true) {
8             IExample newExample = hotswap.swap(oldExample);
9             String message = newExample.message();
10             int count = newExample.plus();
11             System.out.println(message.concat(" : " + count));
12             oldExample = newExample;
13             Thread.sleep(5000);
14         }
15     }
16

利用hotswap替换��q��Example�Q�每5�U�钟轮询一�ơ，swap�Ҏ��实现如下�Q?/div>

1  private IExample swap(IExample old) {
2         try {
3             String sourceFile = srcPath().concat("Example.java");
4             if (isChanged(sourceFile)) {
5                 comiple(sourceFile, classPath());
6                 MyClassLoader classLoader = new MyClassLoader(new URL[]{new URL("file:"+classPath())});
7                 Class clazz = classLoader.loadClass("Example");
8                 System.out.println(IExample.class.getClassLoader());
9                 IExample exampleInstance = ((IExample) clazz.newInstance()).copy(old);
10                 System.out.println(exampleInstance.getClass().getClassLoader());
11                 return exampleInstance;
12             }
13         } catch ...
24         return old;
25     }

�q�里必须��exampleInstance转型为IExample接口而不是Exmaple�Q�否则会抛出ClassCastExecption�Q�这是因为swap�Ҏ��所在类Hotswap是有AppClassLoader加蝲的，而且加蝲Hotswap的同时会加蝲该类引用的Exmaple的symbol link�Q�而Example是MyClassLoader加蝲的，不同的ClassLoader加蝲的类之间直接用会抛出ClassCastException, 在本例中ClassLoader实现如下�Q?/div>

1 public class MyClassLoader extends URLClassLoader {
2
3     public MyClassLoader(URL[] urls) {
4         super(urls);
5     }
6
7     @Override
8     public Class loadClass(String name) throws ClassNotFoundException {
9         if ("Example".equals(name)) {
10             return findClass(name);
11         }
12         return super.loadClass(name);
13     }
14 }

而对IExample我们�q�是调用super的loadClass�Ҏ��Q�该�Ҏ��实现仍是JVM的parent-child委派方式�Q�因此最�l�由AppClassLoader加蝲�Q�加载Hotswap时加载的symbol link也是由AppClassLoader加蝲的，因此能够成功�?/div>

此外再热部��v�Ӟ��被替换的�cȝ��所有引用及状态都要迁�U�d��新的�c�M��Q�本例中只是很简单的调用copy函数�q�移了count的状态�?/div>

Tomcat的jsp热部�|�机制就是基于ClassLoader实现的，对于其类的热部��v机制是通过修改内存中的class字节码实现的�?/div>

Resource�Q?/div>
Reloading Java Classes 101: Objects, Classes and ClassLoaders

Internals of Java Class Loading

*** 2012-09-08 17:58 发表评论

Java Runtime exec问题

*** — Wed, 05 Sep 2012 14:43:00 GMT
1. java.lang.IllegalThreadStateException: process hasn't exited

1 public static void main(String[] args) {
2         try {
3             Process process = Runtime.getRuntime().exec("javac");
4             System.out.println(process.exitValue());
5         } catch (IOException e) {
6             e.printStackTrace();
7         }
8     }

exec�Ҏ��创徏了一个native的进�E�，�q�返回该process的对象，如果�q�程�q�没有返回，调用exitValue�Ҏ��׃��出现此异常，因�ؓ该方法没有阻塞，其实现如下：
1 public synchronized int exitValue() {
2         if (!hasExited) {
3             throw new IllegalThreadStateException("process hasn't exited");
4         }
5         return exitcode;
6     }

2. waitFor�Ҏ��

1 public static void main(String[] args) {
2         try {
3             Process process = Runtime.getRuntime().exec("javac");
4             int result = process.waitFor();
5             System.out.println(result);
6         } catch (IOException e) {
7             e.printStackTrace();
8         } catch (InterruptedException e) {
9             e.printStackTrace();
10         }
11     }

waitFor�Ҏ��会一直阻塞直到native�q�程完成�Q��ƈ�q�回native�q�程的执行结果。如果native�q�程无法执行完成�Q�waitFor�Ҏ��一直阻塞下去，其实现如下：
1 public synchronized int waitFor() throws InterruptedException {
2         while (!hasExited) {
3             wait();
4         }
5         return exitcode;
6     }

该程序在jdk1.7 windows下测试工作正常，�q�回2; 但是jdk1.4 windows下测试出现hang。JDK documention的解释是
The methods that create processes may not work well for special processes on certain native platforms,
such as native windowing processes, daemon processes, Win16/DOS processes on Microsoft Windows,or shell scripts.
The created subprocess does not have its own terminal or console. All its standard io (i.e. stdin, stdout, stderr)
operations will be redirected to the parent process through three streams (getOutputStream(), getInputStream(),
getErrorStream()). The parent process uses these streams to feed input to and get output from the subprocess. Because some native platforms only provide limited buffer size for standard input and output streams,
failure to promptly write the input stream or read the output stream of the subprocess may cause
the subprocess to block, and even deadlock.

所以，出现hang�Ӟ��及时的flush标准输入输出或者错误流能够消除hang�Q�如上面的javac�Q�我们知道redirect到stderr中，所以解决hang后的代码
1 public static void main(String[] args) {
2         try {
3             Process process = Runtime.getRuntime().exec("echo 'abc'>b.txt");
4             BufferedReader reader = new BufferedReader(new InputStreamReader(process.getErrorStream()));
5             String line;
6             while((line=reader.readLine())!=null){
7                 System.out.println(line);
8             }
9             int result = process.waitFor();
10             System.out.println(result);
11         } catch (IOException e) {
12             e.printStackTrace();
13         } catch (InterruptedException e) {
14             e.printStackTrace();
15         }
16     }

3. exec() is not a command line �q�不是所有的command line命��o都可以用exec

1  public static void main(String[] args) {
2         try {
3             Process process = Runtime.getRuntime().exec("echo 'abc'>a.txt");
4             BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
5             String line;
6             while((line=reader.readLine())!=null){
7                 System.out.println(line);
8             }
9             int result = process.waitFor();
10             System.out.println(result);
11         } catch (IOException e) {
12             e.printStackTrace();
13         } catch (InterruptedException e) {
14             e.printStackTrace();
15         }
16     }
�l�果为：
1 'abc'>a.txt
2 0
�q�没有将创徏a.txt�Q�而从命��o行执�?span style="color: #000000; ">"echo 'abc'>a.txt"却正��创��Z��a.txt

*** 2012-09-05 22:43 发表评论

Practices of Extreme Programming - XP

*** — Thu, 03 Nov 2011 13:44:00 GMT
Extreme programming is a set of simple and concrete practices than combine into an agile development process.
Whole Team - Cutomers, managers and developers work closely with one another. So they are all aware of one another's problem and are collaborating to solve the problem. The customers can be a group of BAs, QAs or marketing persons in the same company as developers. The customers can be the paying customer. But in an XP project, the customer, however defined, is the member of and available to the team.
User Stories - In order to plan a project, we must know something about the requirement, but we don't need to know very much. We need to know only enough about a requirement to estimate it. You may think that in order to estimate the requirement, you need to know all the details. That's not quite true. You have to know that there are details and you have to know roughly the kinds of details, but you don't have to know the sepecifics. Since the sepecific details of the requirements are likely to change with time, especially once the customers begin to see the system together. So when talking about requirements with the customer, we write some key words on a card to remind us of the conversation. A user story is memonic token of an ongoing conversation about a requirement. Every user story should have an estimation.
Short Cycles - An XP project delivers working software every iteration - one or two weeks. At the end of each iteration, the sytem is demonstrated to the customer or the stakeholder in order to get their feedbacks. Every release there is a major delivery that can be put into production.
The Planning Game - The customers decide how important a feature is and the developers decide how much effort the feature will cost to implement. So the customers will select a collection of stories based on the priority and the previous iterations' velocity.
The Iteration Plan - Once an iteration has been started, the business agrees not to change the definition or priority of the stories in that iteration. The order of the stories within the iteration is a technical decision. The developers may work on the stories serially or concurrently. It depends but it's a technical decision.
The Release Plan - A release is usually three months. Similarly, the business or customer selecte collections of user stories and determines the priority based on their buget. But release are not cast in stone. The business can change or reorder at any time. They can write new stories, cancel stroies or change the priority. However, the business should not to change an interation that has been started.
Acceptance Tests - The detail about the user stories are captured in the form of acceptance tests specified by the customer. The acceptance tests are written by BAs and QAs immediately before or concurrently with the implementation of that story. They are written in a scripting form that allow them to be run automatically and repeatlly. These tests should be easy to read and understand for customers and business people. These tests become the true requirement document of the project. Once the acceptance test passes, it will be added the body of passing acceptance tests and is never allowed to fail. So the system is migrated from one working state to another.
Pair Programming - One memeber of each pair drives the keyboard and types the code. The other member of the pair watches the code being typed, finding errors and improvements.
The roles change frequently. If the driver gets tired or stuck, the pair grabs the keyboard and starts to drive. The keybord will move back and forth between them serval times in one hour. The resulting code is designed and authored by both memebers.
Pair memebership changes frequently. A reasonable goal is to change pair partner at least once per day. They should have worked on about everything that was going on in this iteration. Also it is very good for knowledge transfer.
Collective Ownership - A pair has right to check out any module and imporve it. No particular pair or developer are individually responsible for one particular module. Every developer of the project has fully responsible for each line of the code. So don't complain about the code may be written by the other pair. Let's try to improve it. Since it is our code.
Open Workspace - The team works together at one table or in one room. Each pair is within earshot of every other pair. Each has the opportunity to hear when another pair is in trouble. Each konws the state of the other. The sound in this room is a buzz of conversation. One might think that this would be a noise and distracting envrionment. But study suggested, working in a "war room" envrionment may increase productivity.
Simple Design - An XP team makes its designs as simple and expressive as they can be. Furthermore, the team narrows its focus to consider only the stories that are planned for the current iteration, not worrying about stories to come. The team migrates the design of the system from iteration to iteration to be the best design for the stories that the system currently implements.
consider the simplest thing that could possibly work. - XP teams always try to find the simplest possible design option fot the current batch of stories.
You aren't going to need it. - That means an XP team introduces one technology or one infrastructure before it is strictly needed. Yeah, but we know we are going to need that database one day. We are going to have to support multiple threads one day. So don't we need to put the hooks in for those things? The team puts the technology or infrastucture in only if it has proof or at least very compelling evidence. that putting it in now will be more cost-effective than waiting.
Once and only once. XPers don't tolerate duplication of code. Wherever they find it. they remove it. The best way to remove redundancy is to create abstractions. After all, if two things are similar, some abstraction can be from them.
Continuous Integration
Sustainable Pace
Test-Driven Development
Refactoring
Metaphor

*** 2011-11-03 21:44 发表评论

*** — Sun, 30 Oct 2011 14:27:00 GMT

�W�一范式 - 列的原子性，数据库表中的每一列都是不可再分的基本数据��?/li>
单一字段有多个有意义的��|��比如 �Q�people�Q�address�Q�其中address包括street�Q�city�Q�country�Q�以逗号分割。想要查询住在某一city的people很不�Ҏ��
用很多字�D�|��表示同一事实�Q�比如（people�Q�address1�Q�address2�Q�address3�Q�，��q��我们假设每个people最多有三个地址�Q�当惌��查询住在同一地址的people时也很不�Ҏ��Q�因为有可能people1的address1与people2的address2相同�Q�每一�ơ都要比�?*3�ơ组�?br />
�W�二范式 - �W�合�W�一范式�Q�且表中的属性必��d��全依赖于全部主键�Q�消除非��d��性对主键的部分依�?/li>
比如�Q�组件ID�Q�供应商ID�Q�供应商姓名�Q��h��|��Q�组件ID+供应商ID��Z��键，��h��完全依赖于全部主键，因�ؓ不同�l��g不同供应商具有不同�h��|��但是对于供应商姓名，则只依赖于供应商ID�Q�会造成对同一供应商ID�Q�名字数据重复，而且如果供应商改名，需要修改全部数据。因此需要单独一个表�Q�供应商ID�Q�供应商姓名�Q�，�Q�组件ID�Q�供应商ID�Q��h��|��
�W�三范式 - 非主属性之间不能有依赖关系�Q�必��都直接依赖于主属性，消除传递依�?/li>
比如�Q�组件ID�Q�制造商姓名�Q�制造商地址�Q�，其中�l��gID��Z��键，而制造商地址依赖于制造商姓名�Q�需要（�l��gID�Q�制造商姓名�Q�和新表�Q�制造商姓名�Q�制造商地址�Q�其中姓名是主键
比如�Q�订单ID�Q�组件ID�Q�单��P��数量�Q��M�h�Q�，其中��M�h=单�h*数量�Q��M�h依赖于单价和数量�Q�需要去掉��M�h�?br />
BC范式 - ��M��属性（包括非主属性和��d��性）不能被非��d��性所军_��。第三范式强调非��d��性不能依赖于其他非主属性，BC范式是第三范式的加强�Q�强�?#8220;��M��属�?#8221;。因此如果满��第三范式，�q�且只有一个主键，则一定满��BC范式
一般，范式��高�Q�表��多�Q�数据库操作旉��要表兌��Q�增加了查询的复杂性，降低了查询性能。因此�ƈ不是范式��高��好�Q�要�Ҏ��需要进行权衡，�W�三范式已经消除了大部分的数据冗余，插入异常�Q�更新异常和删除异常�?br />

*** 2011-10-30 22:27 发表评论

Page Object Patter For Functional Test

*** — Tue, 18 Oct 2011 13:57:00 GMT

Why?
Functional test characteristic
The number of tests is mostly large.
Page structure and elements may be changed frequently.

If our tests interact directly with the test driver (selenium,etc).
UI element locators(Xpath, CSS, element Ids, names, etc) are copuled and repeat throughout the tests logic. It is hard to maintain, refactor and change especially when we change the page structure.
The fine-grained UI operations hide our actual test intention. After some time we can not easily to identify what we want to do before.

What?
A Page Object models the UI elements that your tests interact with as objects within the test code. It decouples the test logic from the fine-grained details of the UI page.

Test -> Page Object -> Test Driver

The driver is the actual executor of browser action, like click, select, type etc. The page object has knowledage of the HTML structure.

Advantages
When UI changes, only responding page object need to be changed. The tests logic will not be affected.
It makes our tests logic simpler and more readable. Since the tests logic can only focus on its test secinaro rather than the structure of HTML page. As an example, think of login function, it only needs username and password, then do login action. How these are implemented shouldn't matter to the test. We don't care about that it uses a button or link to login.
Complex interactions can be modeled as methods on the page object, which can be used in multiple times.

Best Practices

Public methods represent the services that the page offers
Try not to expose the internals of the page. Like OO programming, object just expose the behaviour and hide its internal details.
Generally don't make assertions, but it is better to do some kind of check to ensure the browser is actually on the page it should be. Since our following tests are all base on the assumption. It can be done by checking something simple like title.
Methods return other page object. This means that we can effectively model the user's journey through our application.
Don't need to represent the entire page. It can be just a tab or a navigation bar that your test interacts with.
Only one place knows the HTML structure of a particular page. UI changes, the fix need only be applied in one place.
Different results for the same action are modeled as different methods. Since the test knows the expected state.

*** 2011-10-18 21:57 发表评论

Function - Clean Code

*** — Thu, 25 Aug 2011 00:45:00 GMT
1. Small and the indent level should not be greater than one or two.
2. Do One Thing - Functions should do one thing. They should do it well. They should do it only.
    Steps of function are one level of abstraction below the name of the function. Then the function is doing one thing.
    Another way to know function is doing more than one thing is if you can extract another function from it.
3. One level of abstraction per function
    The stepdown rule - reading code from top to bottom.
    switch
4. Use Descriptive Names
    Don't be afraid to make a name long.
    A long descriptive name is better than a short magic name.
    A long descriptive name is better than a long descriptive comment.
    Dont't be afraid to spend time choosing a name.
    Be consistent in your names.
5. Function Arguments
    The ideal number of arguments for a function is zero.
    Three arguments should be avoided.
    Flag Arguments - can be split into two function.
    Aruments Objects - group the variables and abstract concept if possible.
    Verbs and keywords - function and argument should form a very nice verb/noun pair.
6. Have no side effects
7. Command Query Separation - Functions should either do something or answer something, but not both.
8. Prefer Exceptions to returning error codes.
    Extract try/catch blocks, error handling is one thing.
9. DRY - don't repeat yourself - especially the duplicated logic.

*** 2011-08-25 08:45 发表评论

*** — Mon, 18 Jul 2011 12:09:00 GMT
一�Q�全局变量
1.全局变量可以被�Q何部分在��L��旉��改变�Q�复杂化�Q�降低可靠�?br />2.可能与子�E�序变量名相同，冲突可能��D��E�序无法�q�行�Q�难以调�?br />
三种声明全局变量模式
1.��q��M��函数的var foo �Q?value
2.直接��d��属性至全局对象�Q�全局对象是所有全局变量的容器，在web��览器中全局对象名�ؓwindow�Q�window.foo = value
3.直接未经声明的变�?�Q?隐式的全局变量 foo �Q?value

二，没有块作用域�Q�有函数作用�?br />
函数中定义的参数和变量在函数外部不可见，而在一个函��C��M��位置的定义的变量在该函数的�Q何地方可见�?br />
function f1(){
    var a=1;
    function f2(){
        if(false){
            var a=2;    //变量��Z��函数�Q�而非��Z��语句块，没有块作用域
        }
        console.log(a); //undefined 未定�?br />    }
    f2();
    console.log(a);//1
}
f1();

大多数语�a�中，一般声明变量都是在�W�一�ơ用到它的地方，在javascript中是一个坏习惯�Q�因为没有块作用域，更好的是在每个函数开头声明所有变量�?br />
三，return语句�q�回一个��|��D��辑ּ�必须和return在同一行上

return {
    status:true
};

�q�回包含status成员元素的对象�?br />
如果�?br />
return
{
    status:true
};

会返回undefined�?br />
四，保留字不能用来命名变量或函数�Q�当保留字被用作对象字面量的键值时�Q�必��ȝ��引号�Q�而且不能用点表示法，必须使用括号表示法�?br />
var object = {case:value}; //非法
var object = {'case':value}; //ok
object.case = value;       //非法
object['case'] = value;    //ok

各浏览器对保留字使用限制上实��C��同，以上语法在FF中合法，但其他浏览器不合法；再有不同的保留字行�ؓ也不相同。类似int/long/float�{�保留字在各��览器中都可以做变量名及对象字面量的键倹{��但不徏议��用�Q何保留字�?br />
五，typeof �Q?识别�q�算数类型的字符�?br />
但typeof null�q�回'object'�Q�更好的��null的方法： my_value === null

对于正则表达�?typeof /a/�Q�一些浏览器�q�回'object'�Q�另一些返�?function'

六，parseInt �Q?��字�W�串转换为整�?br />
1.此函数遇到非数字时就停止解析�Q�即parseInt('16')和parseInt('16 abc')产生�l�果相同�Q�都�?6.
2.如果字符串第一个字�W�是0�Q�则��Z��八进制解析，而不是十�q�制。八�q�制中没�?�Q?数字�Q�所以parseInt('08')和parseInt('09')�l�果�?。此函数可以接收一个基��C��为参敎ͼ�parseInt('08',10)�l�果�?�?br />
七，��点�?br />
0.1+0.2不等�?.3

八，NaN

function isNumber(value){
    return typeof value === 'number' && isFinite(value);
}

isFinite�{�调NaN和Infinity�Q�但是isFinite会试图把它的�q�算数�{换�ؓ一个数字，如果�q�算��C��实上不是一个数字，��׃��报错�Q�所以加上typeof value === 'number'�?br />
九，假�?br />
0�Q�NaN�Q?'�Q�false�Q�null�Q�undefined全部�{�于假，但它们是不可互换的�?img src ="http://www.aygfsteel.com/Jennifer/aggbug/354560.html" width = "1" height = "1" />

*** 2011-07-18 20:09 发表评论

*** — Mon, 11 Jul 2011 00:27:00 GMT
印象中美国�h�q�求自由�Q�但他们开会时却是严肃认真�Q�说到开会，他们有一本厚厚的开会规则－�|�伯特议事规则。作者亨�?马丁.�|�伯�Ҏ��业于西点军校�Q�有一�ơ奉命主持一个地�Ҏ��会的会议�Q�结果�h们争个不亦乐乎，什么结论都没有�Q�搞砸啦。于是罗伯特研究��Z��一些规则，最初只有四五条�Q�放在钱包里�Q�开会时遵守�q�些规则�Q�就能顺利成功。后�?876�q�罗伯特写了《通用议事规则》。至今发行五百万册，�|�伯特议事规则已取得最�q�泛的认可和应用�Q�它的根本意囑֜�于��一个会议能以尽可能最有效的方式达到它的目的�?br />
话说�|�伯特议事规则初��C��国是在组�l�农村开会时�Q�当时会议及其�؜乱，表现为：
一�Q�你说周��C�u�Q�我引出周润发，东一榔头西一��子�Q�最后忘了开会的目的是啥�Q�还可能对于已经讨论表决的问题，又重新拿来说�Q�反反复复，归结��h��是跑题�Q?br />二，领导或者权威讲个不停，下面人睡着啦，归结��Z��家言�Q?br />三，争论��h��Q�互相指责，甚至��n��d��Q�归�l��ؓ野蛮争论�?br />
其实�q�三条也是我们日�怼�议经常出现的问题�?br />
首先说野蛮争论问题，很容易解冻I��|�伯特议事规则限制只对会议讨论的问题发言�Q�不允许评议个�h�Q�发�a�时对��L��行陈�q�ͼ�而非辩论�Ҏ��。�ȝ��h��Q�用中国�a�语来说就�?#8220;对事不对�?#8221;�Q�因为指责个人�ƈ不能解决问题�Q�不允许指责一个会员的动机�Q�动机很难衡量，但可以用强烈的语�a�谴责一个议案的性质或后果�?br />
�W�二对于一家言问题�Q�罗伯特议事规则限制每个人发�a�的次数和旉��用以避免一家言问题。其实这是个人智慧和�l�验与集体合作的关系�Q�仍用中国言语来�?#8220;三个臭皮匠顶个诸葛亮”。对于睡着问题�Q�可能是�׃��发言人太��p��Q�可以通过前述�Ҏ��解决�Q�还可能是由于当前话题不感兴��甚��x��养I��则可直接��d��会议或不参加�?br />
�W�三对于跑题问题�Q�罗伯特议事规则规定会议讨论的内容应当是一个明��的动议�Q?#8220;动议�Q�动议，��是行动的徏议！”动议必须是具体的�Q�明��的�Q�可操作的行动徏议，每一个动议都会导致一个action。比�?#8220;早上9点上�?#8221;�Q�而不�?#8220;早上几点上班�Q?#8221;。发�a�人应首先表明观点�Q�赞成还是反对，�q��q�理由。与本次会议动议无关的其他动议，应组�l�其他会议，��x��们常说的“一事一�?#8221;。经双方轮流发言后，��L��人组�l�投��。对于已�l�表决的观点�Q�除非理由充分，�q�经所有成�?/3同意再议�Q�再�l�织再次讨论�?br />
对于�W�三条，我觉得首先需要把�q�x��会议划分一下几�c�：
1.session�Q�做session的�h事先准备好内容，�q�计划和把握session旉��Q�同样中间遇��C��session无关的其他问题或者非常detail的问题，做session的�h要打断讨论，对于重要问题�l�织其他session�Q�对于detail问题offline讨论�?br />2.discussion�Q�如�|�伯特议事规则所��_��l�织discussion的�h要有明确的动议，支持方与反对方轮��发�a��Q�如果规定时间未达成一��_��l�织者须�l�织投票�?br />3.brainstorm�Q�通过集思广益、发挥集体智慧，�q�速地获得大量的新设想与创意，�q�是一�U�极为有效的开启创新思维的方法。头脑风暴结束后才开始对观点�q�行讨论�Q�那针对每一个讨论，又是一个discussion�?br />
另外�Q��ƈ不是所有会议都必须所有�h参加�Q�如果感觉对自己影响不大或者有比会议更要紧的事情可以不参加�Q�或中间��d��?img src ="http://www.aygfsteel.com/Jennifer/aggbug/354071.html" width = "1" height = "1" />

*** 2011-07-11 08:27 发表评论

ORA-00257 archiver error. Connect internal only, until freed

*** — Thu, 02 Jun 2011 14:48:00 GMT
1. Change to oracle user�Q?su oracle
2. Log in as sys user: $sqlplus sys/password as sysdba
3. Find out flash recovery area usage info, from where we can find the archive log space usage percent: SQL>select * from V$FLASH_RECOVERY_AREA_USAGE;
4. Caculate the space used by flash recovery area:SQL>select sum(percent_space_used)*3/100 from V$FLASH_RECOVERY_AREA_USAGE;
5. Show Archive log location: SQL>show parameter log_archive_dest;
6. Show recovery area location: SQL>show parameter recover;
7. Delete or move some old archive log files: SQL>rm -rf archive_log_2011_06_0*;
Notice: After step 7, we must use RMAN to maintain the control files, otherwise, the deletion changes can not be apply.
8. Log in as RMAN: SQL>rman target sys/password;
9. Find out unused archive log: SQL>crosscheck archive log all;
10. Delete expired archive log files: SQL>delete expired archivelog all;
11. Check the flash recovery area usage info again, make sure archive log files have been deleted: SQL>select * from V$FLASH_RECOVERY_AREA_USAGE;
12. Finally, increase the recovery file destination size: SQL>alter system set db_recovery_file_dest_size=3G scope=both;

*** 2011-06-02 22:48 发表评论