反射 学习笔记

于 2018-06-02 08:09:08 发布
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文章标签： java 移动开发
原文链接：https://juejin.im/post/5b11ff48e51d4506e17481d0
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反射

Java反射机制是在运行状态中，对于任何一个类，都可以知道该类的所有属性和方法。对于任何一个类都可以访问该类的所有属性和方法。这种动态获取的信息以及动态调用对象的方法的功能称为java语言的反射机制。

JVM

我们编写的.java文件在被编译器编译之后会生成.class问价，然后被JVM加载生成Class对象。Class是描述每个对象的说明书。

获取Class

反射的入口是Class。

Object.getClass()

在可以获得对象的情况下直接调用getClass()方法即可获得Class对象。

.class 标识

通过类的class属性获得Class对象。

Class.forName() 方法

当无法访问一个类的时候，例如Android系统把一些类加上了@hide注解，所示这些类就不会出现在 Android SDK中。可以通过类的全限定名来获取Class对象。

Class名字

可以根据需要选择适当的API进行类名的获取。

Class修饰符

通过调用 Class.getModifiers()方法可以获取对应属性和方法的修饰符。

输出为 private static。 

public class Modifier {

    /*
     * Bootstrapping protocol between java.lang and java.lang.reflect
     *  packages
     */
    static {
        sun.reflect.ReflectionFactory factory =
            AccessController.doPrivileged(
                new ReflectionFactory.GetReflectionFactoryAction());
        factory.setLangReflectAccess(new java.lang.reflect.ReflectAccess());
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code public} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code public} modifier; {@code false} otherwise.
     */
    public static boolean isPublic(int mod) {
        return (mod & PUBLIC) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code private} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code private} modifier; {@code false} otherwise.
     */
    public static boolean isPrivate(int mod) {
        return (mod & PRIVATE) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code protected} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code protected} modifier; {@code false} otherwise.
     */
    public static boolean isProtected(int mod) {
        return (mod & PROTECTED) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code static} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code static} modifier; {@code false} otherwise.
     */
    public static boolean isStatic(int mod) {
        return (mod & STATIC) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code final} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code final} modifier; {@code false} otherwise.
     */
    public static boolean isFinal(int mod) {
        return (mod & FINAL) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code synchronized} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code synchronized} modifier; {@code false} otherwise.
     */
    public static boolean isSynchronized(int mod) {
        return (mod & SYNCHRONIZED) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code volatile} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code volatile} modifier; {@code false} otherwise.
     */
    public static boolean isVolatile(int mod) {
        return (mod & VOLATILE) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code transient} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code transient} modifier; {@code false} otherwise.
     */
    public static boolean isTransient(int mod) {
        return (mod & TRANSIENT) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code native} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code native} modifier; {@code false} otherwise.
     */
    public static boolean isNative(int mod) {
        return (mod & NATIVE) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code interface} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code interface} modifier; {@code false} otherwise.
     */
    public static boolean isInterface(int mod) {
        return (mod & INTERFACE) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code abstract} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code abstract} modifier; {@code false} otherwise.
     */
    public static boolean isAbstract(int mod) {
        return (mod & ABSTRACT) != 0;
    }

    /**
     * Return {@code true} if the integer argument includes the
     * {@code strictfp} modifier, {@code false} otherwise.
     *
     * @param   mod a set of modifiers
     * @return {@code true} if {@code mod} includes the
     * {@code strictfp} modifier; {@code false} otherwise.
     */
    public static boolean isStrict(int mod) {
        return (mod & STRICT) != 0;
    }

    public static String toString(int mod) {
        StringBuilder sb = new StringBuilder();
        int len;

        if ((mod & PUBLIC) != 0)        sb.append("public ");
        if ((mod & PROTECTED) != 0)     sb.append("protected ");
        if ((mod & PRIVATE) != 0)       sb.append("private ");

        /* Canonical order */
        if ((mod & ABSTRACT) != 0)      sb.append("abstract ");
        if ((mod & STATIC) != 0)        sb.append("static ");
        if ((mod & FINAL) != 0)         sb.append("final ");
        if ((mod & TRANSIENT) != 0)     sb.append("transient ");
        if ((mod & VOLATILE) != 0)      sb.append("volatile ");
        if ((mod & SYNCHRONIZED) != 0)  sb.append("synchronized ");
        if ((mod & NATIVE) != 0)        sb.append("native ");
        if ((mod & STRICT) != 0)        sb.append("strictfp ");
        if ((mod & INTERFACE) != 0)     sb.append("interface ");

        if ((len = sb.length()) > 0)    /* trim trailing space */
            return sb.toString().substring(0, len-1);
        return "";
    }

    /*
     * Access modifier flag constants from tables 4.1, 4.4, 4.5, and 4.7 of
     * <cite>The Java&trade; Virtual Machine Specification</cite>
     */

    /**
     * The {@code int} value representing the {@code public}
     * modifier.
     */
    public static final int PUBLIC           = 0x00000001;

    /**
     * The {@code int} value representing the {@code private}
     * modifier.
     */
    public static final int PRIVATE          = 0x00000002;

    /**
     * The {@code int} value representing the {@code protected}
     * modifier.
     */
    public static final int PROTECTED        = 0x00000004;

    /**
     * The {@code int} value representing the {@code static}
     * modifier.
     */
    public static final int STATIC           = 0x00000008;

    /**
     * The {@code int} value representing the {@code final}
     * modifier.
     */
    public static final int FINAL            = 0x00000010;

    /**
     * The {@code int} value representing the {@code synchronized}
     * modifier.
     */
    public static final int SYNCHRONIZED     = 0x00000020;

    /**
     * The {@code int} value representing the {@code volatile}
     * modifier.
     */
    public static final int VOLATILE         = 0x00000040;

    /**
     * The {@code int} value representing the {@code transient}
     * modifier.
     */
    public static final int TRANSIENT        = 0x00000080;

    /**
     * The {@code int} value representing the {@code native}
     * modifier.
     */
    public static final int NATIVE           = 0x00000100;

    /**
     * The {@code int} value representing the {@code interface}
     * modifier.
     */
    public static final int INTERFACE        = 0x00000200;

    /**
     * The {@code int} value representing the {@code abstract}
     * modifier.
     */
    public static final int ABSTRACT         = 0x00000400;

    /**
     * The {@code int} value representing the {@code strictfp}
     * modifier.
     */
    public static final int STRICT           = 0x00000800;

    // Bits not (yet) exposed in the public API either because they
    // have different meanings for fields and methods and there is no
    // way to distinguish between the two in this class, or because
    // they are not Java programming language keywords
    static final int BRIDGE    = 0x00000040;
    static final int VARARGS   = 0x00000080;
    static final int SYNTHETIC = 0x00001000;
    static final int ANNOTATION  = 0x00002000;
    static final int ENUM      = 0x00004000;
    static final int MANDATED  = 0x00008000;
    static boolean isSynthetic(int mod) {
      return (mod & SYNTHETIC) != 0;
    }

    static boolean isMandated(int mod) {
      return (mod & MANDATED) != 0;
    }

    // Note on the FOO_MODIFIERS fields and fooModifiers() methods:
    // the sets of modifiers are not guaranteed to be constants
    // across time and Java SE releases. Therefore, it would not be
    // appropriate to expose an external interface to this information
    // that would allow the values to be treated as Java-level
    // constants since the values could be constant folded and updates
    // to the sets of modifiers missed. Thus, the fooModifiers()
    // methods return an unchanging values for a given release, but a
    // value that can potentially change over time.

    /**
     * The Java source modifiers that can be applied to a class.
     * @jls 8.1.1 Class Modifiers
     */
    private static final int CLASS_MODIFIERS =
        Modifier.PUBLIC         | Modifier.PROTECTED    | Modifier.PRIVATE |
        Modifier.ABSTRACT       | Modifier.STATIC       | Modifier.FINAL   |
        Modifier.STRICT;

    /**
     * The Java source modifiers that can be applied to an interface.
     * @jls 9.1.1 Interface Modifiers
     */
    private static final int INTERFACE_MODIFIERS =
        Modifier.PUBLIC         | Modifier.PROTECTED    | Modifier.PRIVATE |
        Modifier.ABSTRACT       | Modifier.STATIC       | Modifier.STRICT;


    /**
     * The Java source modifiers that can be applied to a constructor.
     * @jls 8.8.3 Constructor Modifiers
     */
    private static final int CONSTRUCTOR_MODIFIERS =
        Modifier.PUBLIC         | Modifier.PROTECTED    | Modifier.PRIVATE;

    /**
     * The Java source modifiers that can be applied to a method.
     * @jls8.4.3  Method Modifiers
     */
    private static final int METHOD_MODIFIERS =
        Modifier.PUBLIC         | Modifier.PROTECTED    | Modifier.PRIVATE |
        Modifier.ABSTRACT       | Modifier.STATIC       | Modifier.FINAL   |
        Modifier.SYNCHRONIZED   | Modifier.NATIVE       | Modifier.STRICT;

    /**
     * The Java source modifiers that can be applied to a field.
     * @jls 8.3.1  Field Modifiers
     */
    private static final int FIELD_MODIFIERS =
        Modifier.PUBLIC         | Modifier.PROTECTED    | Modifier.PRIVATE |
        Modifier.STATIC         | Modifier.FINAL        | Modifier.TRANSIENT |
        Modifier.VOLATILE;

    /**
     * The Java source modifiers that can be applied to a method or constructor parameter.
     * @jls 8.4.1 Formal Parameters
     */
    private static final int PARAMETER_MODIFIERS =
        Modifier.FINAL;

    /**
     *
     */
    static final int ACCESS_MODIFIERS =
        Modifier.PUBLIC | Modifier.PROTECTED | Modifier.PRIVATE;

    /**
     * Return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a class.
     * @return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a class.
     *
     * @jls 8.1.1 Class Modifiers
     * @since 1.7
     */
    public static int classModifiers() {
        return CLASS_MODIFIERS;
    }

    /**
     * Return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to an interface.
     * @return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to an interface.
     *
     * @jls 9.1.1 Interface Modifiers
     * @since 1.7
     */
    public static int interfaceModifiers() {
        return INTERFACE_MODIFIERS;
    }

    /**
     * Return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a constructor.
     * @return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a constructor.
     *
     * @jls 8.8.3 Constructor Modifiers
     * @since 1.7
     */
    public static int constructorModifiers() {
        return CONSTRUCTOR_MODIFIERS;
    }

    /**
     * Return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a method.
     * @return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a method.
     *
     * @jls 8.4.3 Method Modifiers
     * @since 1.7
     */
    public static int methodModifiers() {
        return METHOD_MODIFIERS;
    }

    /**
     * Return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a field.
     * @return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a field.
     *
     * @jls 8.3.1 Field Modifiers
     * @since 1.7
     */
    public static int fieldModifiers() {
        return FIELD_MODIFIERS;
    }

    /**
     * Return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a parameter.
     * @return an {@code int} value OR-ing together the source language
     * modifiers that can be applied to a parameter.
     *
     * @jls 8.4.1 Formal Parameters
     * @since 1.8
     */
    public static int parameterModifiers() {
        return PARAMETER_MODIFIERS;
    }
}

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Modifier 还提供了一系列的静态工具方法用来对修饰符进行操作。

获取 Class 的成员

获取 Filed

//获取所有的属性，但不包括从父类继承下来的属性
public Field[] getDeclaredFields() throws SecurityException {}

//获取自身的所有的 public 属性，包括从父类继承下来的。
public Field[] getFields() throws SecurityException {}
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获取 Method

public Method getDeclaredMethod(String name, Class<?>... parameterTypes)

public Method getMethod(String name, Class<?>... parameterTypes)

public Method[] getDeclaredMethods() throws SecurityException

public Method getMethod(String name, Class<?>... parameterTypes) 
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获取 Constructor

public Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes)

public Constructor<T> getConstructor(Class<?>... parameterTypes)

public Constructor<?>[] getDeclaredConstructors() throws SecurityException 

public Constructor<?>[] getConstructors() throws SecurityException 
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需要注意的是，Constructor 不能从父类继承。

操作Field

获取Field类型
//能够获取到泛型类型
public Type getGenericType() {}
//基础类型
public Class<?> getType() {}
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获取Field类型修饰符
public int getModifiers() {}
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读取与设置Field内容

读取内容

public Object get(Object obj);

public int getInt(Object obj);

public long getLong(Object obj) throws IllegalArgumentException, IllegalAccessException;

public float getFloat(Object obj) throws IllegalArgumentException, IllegalAccessException;

public short getShort(Object obj) throws IllegalArgumentException, IllegalAccessException;

public double getDouble(Object obj) throws IllegalArgumentException, IllegalAccessException;

public char getChar(Object obj) throws IllegalArgumentException, IllegalAccessException;

public byte getByte(Object obj) throws IllegalArgumentException, IllegalAccessException;

public boolean getBoolean(Object obj) throws IllegalArgumentException, IllegalAccessException
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设置内容

public void set(Object obj, Object value);

public void getInt(Object obj,int value);

public void getLong(Object obj,long value) throws IllegalArgumentException, IllegalAccessException;

public void getFloat(Object obj,float value) throws IllegalArgumentException, IllegalAccessException;

public void getShort(Object obj,short value) throws IllegalArgumentException, IllegalAccessException;

public void getDouble(Object obj,double value) throws IllegalArgumentException, IllegalAccessException;

public void getChar(Object obj,char value) throws IllegalArgumentException, IllegalAccessException;

public void getByte(Object obj,byte b) throws IllegalArgumentException, IllegalAccessException;

public void getBoolean(Object obj,boolean b) throws IllegalArgumentException, IllegalAccessException
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Class 本身不对成员进行储存，它只提供检索，所以需要用Field、Method、Constructor 对象来承载这些成员，所以，针对成员的操作时，一般需要为成员指定类的实例引用。

操作Method

获取Method类型
public String getName() {}
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获取Method方法参数
public Parameter[] getParameters() {}
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Parameter.java

// 获取参数名字
public String getName() {}

// 获取参数类型
public Class<?> getType() {}

// 获取参数的修饰符
public int getModifiers() {}

// 获取所有的参数类型
public Class<?>[] getParameterTypes() {}

// 获取所有的参数类型，包括泛型
public Type[] getGenericParameterTypes() {}
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获取Method返回值类型
// 获取返回值类型
public Class<?> getReturnType() {}

// 获取返回值类型包括泛型
public Type getGenericReturnType() {}
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获取Method修饰符
public int getModifiers() {}
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获取Method异常类型
public Class<?>[] getExceptionTypes() {}

public Type[] getGenericExceptionTypes() {}
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执行Method方法
public Object invoke(Object obj, Object... args) {}
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Method 调用 invoke() 的时候，存在许多细节：

1.invoke() 方法中第一个参数 Object 实质上是 Method 所依附的 Class 对应的类的实例，如果这个方法是一个静态方法，那么 ojb 为 null，后面的可变参数 Object 对应的自然就是参数。

2.invoke() 返回的对象是 Object，所以实际上执行的时候要进行强制转换。

3.在对 Method 调用 invoke() 的时候，如果方法本身会抛出异常，那么这个异常就会经过包装，由 Method 统一抛出 InvocationTargetException。而通过 InvocationTargetException.getCause() 可以获取真正的异常。

操作Constructor

在 Java 反射机制中有两种方法可以用来创建类的对象实例：Class.newInstance() 和 Constructor.newInstance()。

1.Class.newInstance() 只能调用无参的构造方法，而 Constructor.newInstance() 则可以调用任意的构造方法。

2.Class.newInstance() 通过构造方法直接抛出异常，而 Constructor.newInstance() 会把抛出来的异常包装到 InvocationTargetException 里面去，这个和 Method 行为一致。

3.Class.newInstance() 要求构造方法能够被访问，而 Constructor.newInstance() 却能够访问 private 修饰的构造器。

操作数组

动态创建数组

Array.newInstance()

public static Object newInstance(Class<?> componentType, int... dimensions)
        throws IllegalArgumentException, NegativeArraySizeException {}
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第一个参数是数组内的元素类型，第一个参数是可变参数，表示的是相应维度的数组长度限制。

Array.newInstance(String.class,2,3);表示创建一个String[2][3]的数组，

读取与赋值数组
public static void set(Object array,
                       int index,
                       Object value)
                throws IllegalArgumentException,
                       ArrayIndexOutOfBoundsException;

public static Object get(Object array,
                         int index)
                  throws IllegalArgumentException,
                         ArrayIndexOutOfBoundsException;
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操作Enum

// 用来判定 Class 对象是不是枚举类型
Class.isEnum()

// 获取所有的枚举常量
Class.getEnumConstants()

// 判断一个 Field 是不是枚举常量
java.lang.reflect.Field.isEnumConstant()
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获取与设定枚举

因为等同于 Class，所以枚举的获取与设定就可以通过 Field 中的 get() 和 set() 方法。

需要注意的是，如果要获取枚举里面的 Field、Method、Constructor 可以调用 Class 的通用 API。

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