Java提供了参数的动态检验，也就是在执行期检测参数是否合法。但是部分的检测还是在编译器的协助下完成的，而这一步要早于真正执行它。Java编译器会开启这种静态检测机制，会使用一组类型检测规则来检测Java字节码，检测这些字节码是否符合规则，如果不符合那么将会被拒绝。这里不讨论类型检测的做法，而是说明类型检测是早于执行的，而这一步将会诱发类加载过程，而这个类加载的顺序将会 颠覆 你对Java类加载的认知。
场景一：方法接受参数
当出现方法接收的参数在代码中出现了参数类型的子类时，那么参数类型和子类，将会优先于方法所在类进行加载。因为Java需要判定这个方法是否可以接收这个类型，着重点在验证方法参数类型之间的关系。
public class A {
}
public class B extends A {
}
public class Test {
void m(A a) {
}
}
public class Main {
public static void main(String[] args) {
Test test = new Test();
test.m(new B());
}
}
按照一般的理解，Java在执行main时，首先加载Test类型，然后再加载A和B两个参数类型，但是实际上的加载顺序是：
[Loaded com.murdock.book.jarviewer.typechecking.method.Main from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.method.A from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.method.B from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Void from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded com.murdock.book.jarviewer.typechecking.method.Test from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Shutdown from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded java.lang.Shutdown$Lock from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
Java先加载了方法的参数类型和返回值类型（Void），然后再加载了Test类型。 因为我们的代码里面传递给m方法的不是A类型，从而诱发Java的编译器检查，进而促使了参数类型的提前加载。下面我们把方法的参数改为A。
test.m(new A());
加载顺序和我们平日认知的一致：
[Loaded com.murdock.book.jarviewer.typechecking.method.Main from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Void from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded com.murdock.book.jarviewer.typechecking.method.Test from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.method.A from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Shutdown from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded java.lang.Shutdown$Lock from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
先加载了Test类型，然后再加载A类型。
场景二：成员变量赋值
当一个类的成员变量被赋值一个子类型时，该成员变量的类型和子类型将会优先于成员变量所在类进行加载。
public class A {
}
public class B extends A {
}
public class Test {
A a = null;
void m(A a) {
}
}
public class Main {
public static void main(String[] args) {
Test test = new Test();
test.a = new B();
}
}
a指向了子类型B的实例，加载顺序：
[Loaded com.murdock.book.jarviewer.typechecking.assign.Main from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.assign.A from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.assign.B from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Void from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded com.murdock.book.jarviewer.typechecking.assign.Test from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Shutdown from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded java.lang.Shutdown$Lock from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
A和B又早于Test进行了加载。
场景三：方法体返回值的类型为签名的子类型
当一个类中包括的方法，返回的类型是形参的子类时，形参和返回参数的子类型将会提前加载。
public class A {
}
public class B extends A {
}
public class Test {
A a = null;
void m(A a) {
}
}
public class Main {
public static void main(String[] args) {
Test test = new Test();
test.m(new A());
}
A check(A a) {
return new B();
}
}
在Main执行过程中，check方法是不会被调用的，但是在Main这个类里面存在一个这个方法，所以当Main被加载之后，Java观察到方法check返回了一个非A的类型，那么就提前加载了A和B类型，加载顺序：
[Loaded com.murdock.book.jarviewer.typechecking.bodyreturn.Main from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.bodyreturn.A from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded com.murdock.book.jarviewer.typechecking.bodyreturn.B from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Void from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded com.murdock.book.jarviewer.typechecking.bodyreturn.Test from file:/Users/weipeng2k/Documents/arena/jarviewer/target/classes/]
[Loaded java.lang.Shutdown from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
[Loaded java.lang.Shutdown$Lock from /Library/Java/JavaVirtualMachines/jdk1.7.0_67.jdk/Contents/Home/jre/lib/rt.jar]
这三种提前加载，都是由于Java编译器在进行规则检查时进行了提前加载所致。
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