Reflection 源碼分析

定義

Java的反射（reflection）機制是指在程序的運行狀態(tài)中，可以構(gòu)造任意一個類的對象，可以了解任意一個對象所屬的類，可以了解任意一個類的成員變量和方法，可以調(diào)用任意一個對象的屬性和方法。這種動態(tài)獲取程序信息以及動態(tài)調(diào)用對象的功能稱為Java語言的反射機制。反射被視為動態(tài)語言的關鍵

Class對象

• Class對象是JVM生成用來保存對象的類的信息的。Java程序執(zhí)行之前需要經(jīng)過編譯、加載、鏈接和初始化這幾個階段，編譯階段會將源碼文件編譯為.class字節(jié)碼文件，編譯器同時會在.class文件中生成Class對象，加載階段通過JVM內(nèi)部的類加載機制，將Class對象加載到內(nèi)存中。在創(chuàng)建對象實例之前，JVM會先檢查Class對象是否在內(nèi)存中存在，如果不存在，則加載Class對象，然后再創(chuàng)建對象實例，如果存在，則直接根據(jù)Class對象創(chuàng)建對象實例。JVM中只有一個Class對象，但可以根據(jù)Class對象生成多個對象實例。

• 獲取Class對象的三種方式

  • 1.Class.forName方式 傳全限定類名

    public class Cat {
        private int age;
        private String name;
    
        public static void main(String[] args) throws ClassNotFoundException {
            Class<?> c1 = Class.forName("org.yrdm.ch1.Cat");
            System.out.println(c1);
        }
    }
    
    結(jié)果：
        class org.yrdm.ch1.Cat
    

  • 2. 調(diào)用類實例對象的getClass()方法，繼承于Object類的getClass()方法

    public class Cat {
        private int age;
        private String name;
    
        public static void main(String[] args) {
            Cat cat=new Cat();
            Class c1 = cat.getClass();
            System.out.println(c1);
        }
    }
    
    結(jié)果：
        class org.yrdm.ch1.Cat
    

  • 3.通過類名 .class

    public class Cat {
        private int age;
        private String name;
    
        public static void main(String[] args) {
            Class c1 = Cat.class;
            System.out.println(c1);
        }
    }
    
    結(jié)果：
        class org.yrdm.ch1.Cat
    

• Class類源碼分析文章來源地址http://www.zghlxwxcb.cn/news/detail-433391.html

  //泛型類
  public final class Class<T> implements java.io.Serializable,GenericDeclaration,Type,AnnotatedElement {
  	//重要方法
      //獲取Class對象 static修飾
      @CallerSensitive
      public static Class<?> forName(String className)
                  throws ClassNotFoundException {
          Class<?> caller = Reflection.getCallerClass();
          return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
      }
                                    
      //創(chuàng)建實例,直接調(diào)用無參構(gòu)造進行構(gòu)造 jdk9后被棄用，使用class.getDeclaredConstructor().newInstance()代替
      @CallerSensitive
      @Deprecated(since="9")
      public T newInstance()
          throws InstantiationException, IllegalAccessException
      {...}
      
      //每個結(jié)構(gòu)都有這四件套 getXXX()  getXXXs()  getDealaredXXX()  getDeclaredXXXs()
      //1.屬性
      //根據(jù)參數(shù)獲取屬性 public 屬性
      @CallerSensitive
      public Field getField(String name){...}
      
      //獲取所有public屬性，返回屬性數(shù)組
      @CallerSensitive
      public Field[] getFields() throws SecurityException {
          ...
      }
      
      //Returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object.  獲取所有屬性，public protected default private
      @CallerSensitive
      public Field[] getDeclaredFields() throws SecurityException {
          ...
      }
      
      //Returns a Field object that reflects the specified declared field of the class or interface represented by this Class object.
      @CallerSensitive
      public Field getDeclaredField(String name)
          throws NoSuchFieldException, SecurityException {
          ...
      }
      
      
      //2.獲取方法
      //(1).getMethod  name為方法名，因為重載的緣故根據(jù)后面的可變長參數(shù)來指定特定的方法
      @CallerSensitive
      public Method getMethod(String name, Class<?>... parameterTypes)
          throws NoSuchMethodException, SecurityException {
          ...
      }
      //(2).getMethods()
      @CallerSensitive
      public Method[] getMethods() throws SecurityException {
          ...
      }
      //(3)getDeclaredMethods()
      @CallerSensitive
      public Method[] getDeclaredMethods() throws SecurityException {
          。。。
      }
      //(4).getDeclaredMethod()
      @CallerSensitive
      public Method getDeclaredMethod(String name, Class<?>... parameterTypes)
          throws NoSuchMethodException, SecurityException {
          ...
      }
      
      //3.獲取構(gòu)造函數(shù)
       @CallerSensitive
      public Constructor<T> getConstructor(Class<?>... parameterTypes)
          throws NoSuchMethodException, SecurityException
      {
          
      }
      
      @CallerSensitive
      public Constructor<?>[] getConstructors() throws SecurityException {
          
      }
      
      @CallerSensitive
      public Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes)
          throws NoSuchMethodException, SecurityException
      {
         ...
      }
      
      @CallerSensitive
      public Constructor<?>[] getDeclaredConstructors() throws SecurityException {
          
      }
      
      //4.獲取所實現(xiàn)的接口
      public Class<?>[] getInterfaces() {
          ...
      }
      
      public Type[] getGenericInterfaces() {
          
      }
      
      //5.獲取父類
      public Type getGenericSuperclass() {
          ...
      }
      
      //6.獲取注解
      public <A extends Annotation> A getAnnotation(Class<A> annotationClass) {
         
      }
  
      public Annotation[] getAnnotations() {
          return AnnotationParser.toArray(annotationData().annotations);
      }
      
      public <A extends Annotation> A getDeclaredAnnotation(Class<A> annotationClass) {
          Objects.requireNonNull(annotationClass);
  
          return (A) annotationData().declaredAnnotations.get(annotationClass);
      }
      
      public Annotation[] getDeclaredAnnotations()  {
          return AnnotationParser.toArray(annotationData().declaredAnnotations);
      }
      
      //7.獲取類中定義的類和嵌套接口
      @CallerSensitive
      public Class<?>[] getClasses() {
          ...
      }
      
      @CallerSensitive
      public Class<?>[] getDeclaredClasses() throws SecurityException {
          SecurityManager sm = System.getSecurityManager();
          if (sm != null) {
              checkMemberAccess(sm, Member.DECLARED, Reflection.getCallerClass(), false);
          }
          return getDeclaredClasses0();
      }
      
      //8.獲取當前類的加載器
      @CallerSensitive
      @ForceInline // to ensure Reflection.getCallerClass optimization
      public ClassLoader getClassLoader() {
          ClassLoader cl = getClassLoader0();
          if (cl == null)
              return null;
          SecurityManager sm = System.getSecurityManager();
          if (sm != null) {
              ClassLoader.checkClassLoaderPermission(cl, Reflection.getCallerClass());
          }
          return cl;
      }
      
      //9.其他
      public Module getModule() {
          return module;
      }
      
      public String getName() {
          String name = this.name;
          return name != null ? name : initClassName();
      }
      
      public Package getPackage() {
          if (isPrimitive() || isArray()) {
              return null;
          }
          ClassLoader cl = getClassLoader0();
          return cl != null ? cl.definePackage(this)
                            : BootLoader.definePackage(this);
      }
      
      public String getPackageName() {
          ...
      }
      //Finds a resource with a given name.
      @CallerSensitive
      public URL getResource(String name) {
          ...
      }
      
      @CallerSensitive
      public InputStream getResourceAsStream(String name) {}
      
  }
  

URL類

• Class.getResource() 返回URL類

public final class URL implements java.io.Serializable {
	/**
     * The protocol to use (ftp, http, nntp, ... etc.) .
     * @serial
     */
    private String protocol;

    /**
     * The host name to connect to.
     * @serial
     */
    private String host;

    /**
     * The protocol port to connect to.
     * @serial
     */
    private int port = -1;

    /**
     * The specified file name on that host. {@code file} is
     * defined as {@code path[?query]}
     * @serial
     */
    private String file;

    /**
     * The query part of this URL.
     */
    private transient String query;

    /**
     * The authority part of this URL.
     * @serial
     */
    private String authority;

    /**
     * The path part of this URL.
     */
    private transient String path;
    /**
     * The userinfo part of this URL.
     */
    private transient String userInfo;

    /**
     * # reference.
     * @serial
     */
    private String ref;
    
    ...
}

代碼轉(zhuǎn)載自：
*****https://geek-docs.com/java/java-url/g_url-class-java-examples.html************



// Java program to demonstrate working of URL
  
// Importing required classes
import java.net.MalformedURLException;
import java.net.URL;
  
// Main class
// URL class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
        throws MalformedURLException
    {
  
        // Creating a URL with string representation
        URL url1 = new URL(
            "https://www.google.co.in/?gfe_rd=cr&ei=ptYq"
            + "WK26I4fT8gfth6CACg#q=geeks+for+geeks+java");
  
        // Creating a URL with a protocol,hostname,and path
        URL url2 = new URL("http", "www.geeksforgeeks.org",
                           "/jvm-works-jvm-architecture/");
  
        URL url3 = new URL(
            "https://www.google.co.in/search?"
            + "q=gnu&rlz=1C1CHZL_enIN71"
            + "4IN715&oq=gnu&aqs=chrome..69i57j6"
            + "9i60l5.653j0j7&sourceid=chrome&ie=UTF"
            + "-8#q=geeks+for+geeks+java");
  
        // Printing the string representation of the URL
        System.out.println(url1.toString());
        System.out.println(url2.toString());
        System.out.println();
        System.out.println(
            "Different components of the URL3-");
  
        // Retrieving the protocol for the URL
        System.out.println("Protocol:- "
                           + url3.getProtocol());
  
        // Retrieving the hostname of the url
        System.out.println("Hostname:- " + url3.getHost());
  
        // Retrieving the default port
        System.out.println("Default port:- "
                           + url3.getDefaultPort());
  
        // Retrieving the query part of URL
        System.out.println("Query:- " + url3.getQuery());
  
        // Retrieving the path of URL
        System.out.println("Path:- " + url3.getPath());
  
        // Retrieving the file name
        System.out.println("File:- " + url3.getFile());
  
        // Retrieving the reference
        System.out.println("Reference:- " + url3.getRef());
    }
}

運行結(jié)果：
    https://www.google.co.in/?gfe_rd=cr&ei=ptYqWK26I4fT8gfth6CACg#q=geeks+for+geeks+java
    https://www.geeksforgeeks.org/jvm-works-jvm-architecture/

    Different components of the URL3-
    Protocol:- https
    Hostname:- www.google.co.in
    Default port:- 443
    Query:- q=gnu&rlz=1C1CHZL_enIN714IN715&oq=gnu&aqs=chrome..69i57j69i60l5.653j0j7&sourceid=chrome&ie=UTF-8
    Path:- /search
    File:- /search?q=gnu&rlz=1C1CHZL_enIN714IN715&oq=gnu&aqs=chrome..69i57j69i60l5.653j0j7&sourceid=chrome&ie=UTF-8
    Reference:- q=geeks+for+geeks+java

ClassLoader

• java 類加載過程（暫時挖個坑，后面填

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