JMockit 是用以帮助开发人员编写测试程序的一组工具和API，该项目完全基于 Java 5 SE 的 java.lang.instrument 包开发，内部使用 ASM 库来修改Java的Bytecode。

Mocking with Jmockit

在单元测试中，经常需要进行一些mock操作。现在已经有了一些比较不错的框架在做这些事情，比如：EasyMck，他在大多数情况下运行良好，但是对于某些结构的设计却显得无能为力。

EasyMock等众多的mock框架仅能mock一些public，non static or final的方法，在大多数情况下这并没有什么问题，他可以处理大多数的问题，但是当测试的代码包含了一些静态方法，可能就让问题变得难以解决，有一种选择即是重构他（过多的静态方法可能预示着这并不是一个很好的设计），但是当你使用外部引用库所提供的方法，问题又该如何解决呢？

JMockit是一个能帮我们解决以上问题的轻量级框架，他允许你动态的改变已有的方法，这主要基于java 1.5的Instrumentation框架，这样便可以使得JMockit能够适应几乎所有的设计。他允许你重定义private,static and final方法，甚至是no-arg constructors都能够并轻易的重定义。

在实际的项目中有些方法可以重定义而有些不行，为了更好的说明如何对方法进行重定义，下面有一个简单的类和对应测试代码的demo，他尽可能考虑到了几乎所有的情况，供大家方便的学习。

ClassToMock :

publicclassClassToMock

privateString memberToSet;

privatestaticString staticMember;

static

staticMember= "Static initialized";

publicClassToMock

this.memberToSet= "Member set by original constructor";

publicClassToMock(String value)

this.memberToSet= "Member set by original constructor";

publicString publicMethod

return"Original public method";

protectedString protectedMethod

return"Original protected method";

String defaultMethod

return"Original default method";

publicString methodThatUsesPrivateMethod

returnprivateMethod;

privateString privateMethod

return"Original private method";

publicString getMemberToSet

returnmemberToSet;

publicString getStaticMember

returnstaticMember。

ClassToMockTest :

importjunit.framework.TestCase;

importmockit.Mockit;

publicclassClassToMockTest extendsTestCase

privateClassToMock mockedClass;

publicstaticclassReplacement

static

publicReplacement

publicReplacement(String test)

publicString publicMethod

return"Replaced public method";

publicString protectedMethod

return"Replaced protected method";

publicString defaultMethod

return"Replaced default method";

publicString privateMethod

return"Replaced private method";

protectedvoidsetUp throwsException

Mockit.redefineMethods(ClassToMock.class, Replacement.class);

mockedClass = newClassToMock("test");

protectedvoidtearDown throwsException

Mockit.restoreAllOriginalDefinitions;

Public methods can be replaced

publicvoidtestReplacePublic

assertEquals("Replaced public method", mockedClass.publicMethod);

Protected methods can be replaced.

The replacement method should be declared public however

publicvoidtestReplaceProtected

assertEquals("Replaced protected method", mockedClass.protectedMethod);

Package accessable methods can be replaced

The replacement method should be declared public however

publicvoidtestReplaceDefault

assertEquals("Replaced default method", mockedClass.defaultMethod);

Private methods can be replaced

The replacement method should be declared public however

publicvoidtestReplacePrivate

assertEquals("Replaced private method", mockedClass

.methodThatUsesPrivateMethod);

Non-default constructors can be replaced

Your mock definition must have a default constructor however

publicvoidtestReplaceConstructor

assertEquals(null, mockedClass.getMemberToSet);

Default constructors <b>can't</b> be replaced

publicvoidtestReplaceDefaultConstructor

mockedClass = newClassToMock;

assertEquals("Member set by original constructor", mockedClass

.getMemberToSet);

Static initializers <b>can't</b> be replaced

publicvoidtestReplaceStaticBlock

assertEquals("Static initialized", mockedClass.getStaticMember);