自定义ClassLoader直接继承ClassLoader抽象类, 覆写
protected Class<?> findClass(String name) throws ClassNotFoundException
方法, 实现自定义加载类. 在 #ClassLoader抽象类中, 给出了 class实例化方法:
protected final Class<?> defineClass(byte[] b, int off, int len)
这个方法目的是将 class字节流转为 class instance
/**
* Converts an array of bytes into an instance of class <tt>Class</tt>.
* Before the <tt>Class</tt> can be used it must be resolved.
*
* <p> This method assigns a default {@link java.security.ProtectionDomain
* <tt>ProtectionDomain</tt>} to the newly defined class. The
* <tt>ProtectionDomain</tt> is effectively granted the same set of
* permissions returned when {@link
* java.security.Policy#getPermissions(java.security.CodeSource)
* <tt>Policy.getPolicy().getPermissions(new CodeSource(null, null))</tt>}
* is invoked. The default domain is created on the first invocation of
* {@link #defineClass(String, byte[], int, int) <tt>defineClass</tt>},
* and re-used on subsequent invocations.
*
* <p> To assign a specific <tt>ProtectionDomain</tt> to the class, use
* the {@link #defineClass(String, byte[], int, int,
* java.security.ProtectionDomain) <tt>defineClass</tt>} method that takes a
* <tt>ProtectionDomain</tt> as one of its arguments. </p>
*
* @param name
* The expected <a href="#name">binary name</a> of the class, or
* <tt>null</tt> if not known
*
* @param b
* The bytes that make up the class data. The bytes in positions
* <tt>off</tt> through <tt>off+len-1</tt> should have the format
* of a valid class file as defined by
* <cite>The Java™ Virtual Machine Specification</cite>.
*
* @param off
* The start offset in <tt>b</tt> of the class data
*
* @param len
* The length of the class data
*
* @return The <tt>Class</tt> object that was created from the specified
* class data.
*
* @throws ClassFormatError
* If the data did not contain a valid class
*
* @throws IndexOutOfBoundsException
* If either <tt>off</tt> or <tt>len</tt> is negative, or if
* <tt>off+len</tt> is greater than <tt>b.length</tt>.
*
* @throws SecurityException
* If an attempt is made to add this class to a package that
* contains classes that were signed by a different set of
* certificates than this class (which is unsigned), or if
* <tt>name</tt> begins with "<tt>java.</tt>".
*
* @see #loadClass(String, boolean)
* @see #resolveClass(Class)
* @see java.security.CodeSource
* @see java.security.SecureClassLoader
*
* @since 1.1
*/
protected final Class<?> defineClass(String name, byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(name, b, off, len, null);
}
这里是从本地文件系统中通过文件的方式读取*.class文件, 将获取到的全部class文件字节流传递给 #defineClass方法, 实例化. 这之后我们就可以使用这个类对象了.
public class MyClassLoader extends ClassLoader {
private String dir ;
public MyClassLoader(String dir) {
//
this.dir = dir ;
}
@Override
public Class<?> findClass(String name) throws ClassNotFoundException {
//
if (StringUtils.isBlank(name))
throw new ClassNotFoundException() ;
String path = this.getPath(name) ;
byte[] ret = this.getClassData(path) ;
if (ret == null)
throw new ClassNotFoundException("读取".concat(path).concat("失败.")) ;
return defineClass(name, ret,0, ret.length) ;
}
private byte[] getClassData(String path) {
System.out.println("Path: - ".concat(path));
try {
InputStream inputStream = new FileInputStream(new File(path)) ;
ByteArrayOutputStream baos = new ByteArrayOutputStream() ;
byte[] buffer = new byte[1024] ;
while (inputStream.read(buffer) != -1) {
// baos.write(buffer);
baos.write(buffer, 0, buffer.length);
}
return baos.toByteArray() ;
} catch (Exception e) {
e.printStackTrace();
}
return null ;
}
private String getPath(String className) {
return this.dir.concat(className.replace(".", File.separator)).concat(".class") ;
}
}
随便定一个使用自定义类加载器加载的类:
public class ABean {
public ABean() {
System.out.println("初始化ABean.");
}
@Override
public String toString() {
return "hello, myClassLoader...." ;
}
}
测试类:
public static void main(String[] args) {
//JVM-classLoader test
String dir = "/test/target/classess/test/jvm/",
clsName = "test.jvm.ABean" ;
MyClassLoader myClassLoader = new MyClassLoader(dir) ;
try {
Class cls = myClassLoader.loadClass(clsName) ;
System.out.println(cls.newInstance().toString()) ;
} catch (Exception e) {
e.printStackTrace();
}
}
//输出
初始化ABean.
hello, myClassLoader....
注意
需要注意的是
protected final Class<?> defineClass(String name, byte[] b, int off, int len)
这个方法第一个参数是指: 类全名[Full-className] 否则会有异常: java.lang.NoClassDefFoundError 这里我根据方法文档, 大概是因为:
* @throws NoClassDefFoundError
* If <tt>name</tt> is not equal to the <a href="#name">binary
* name</a> of the class specified by <tt>b</tt>
在读取class文件得到的byteStream名称为 test/jvm/ABean/class 但是传递给#defineClass方法的名称却是 ABean.
以上代码测试环境为
Linux arch 4.14.10-1-ARCH x86_64 GNU/Linux
触发类加载有以下三种方式：
- 命令行启动应用时候由JVM初始化加载
- 通过Class.forName()方法动态加载
- 通过ClassLoader.loadClass()方法动态加载
类装载器把一个类装入JVM中，要经过以下步骤：
- 装载：查找和导入Class文件；
- 链接：把类的二进制数据合并到JRE中； 这个过程包括三个步骤:
- 校验：检查载入Class文件数据的正确性；
- 准备：给类的静态变量分配存储空间；
- 解析：将符号引用转成直接引用；
- 初始化：对类的静态变量，静态代码块执行初始化操作