Java反射和注解一些总结

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Java 反射和注解的一些总结

反射

// There are five kinds of classes (or interfaces):// a) Top level classes// b) Nested classes (static member classes)// c) Inner classes (non-static member classes)// d) Local classes (named classes declared within a method)// e) Anonymous classes

五种类或接口

顶层类 Top level classes
嵌套类Nested classes / 静态成员类static member classes
内部类Inner classes/ 非静态成员类non-static member classes
局部类Local classes 类的声明在一个方法中
匿名类Anonymous classes

Class对象的方法

Class getEnclosingClass(); //嵌套类、内部类调用，获取其所在的外围类的类字面常量(Class对象)Method getEnclosingMethod(); //局部类、匿名类调用，获取其所在的外围方法Class[] getDeclaredClasses(); //获取当前类的所有声明的内部类newInstance();//实例化对象//公有构造函数Constructor<T> getConstructor(Class<?>... parameterTypes);Constructor[] getCounstructors();//所有声明的构造函数Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes);Constructor[] getDeclaredCounstructors();//获取声明的作用域(不包含父类)Field getDeclaredField(String name); Field[] getDeclaredFields()//获取公有作用域(包含父类)Field getField(String name);Field[] getFields();//获取声明的方法(不包含父类)Method getDeclaredMethod(String name, Class<?>... parameterTypes);Method[] getDeclaredMethods();//获取公有的方法(包含父类)Method getMethod(String name, Class<?>... parameterTypes);Method[] getMethods();public int getModifiers(); //修饰符//声明在该类上的注解 @Target(ElementType.TYPE)<A extends Annotation> A getDeclaredAnnotation(Class<A> annotationClass)Annotation[] getDeclaredAnnotations()

Field对象方法

public void set(Object obj, Object value) //设置作用域的值，obj为目标对象，value为带设置的值public Object get(Object obj) //获取对象在该作用域的值public int getModifiers(); //修饰符//声明在该作用域上的注解 @Target(ElementType.FIELD)<A extends Annotation> A getDeclaredAnnotation(Class<A> annotationClass)Annotation[] getDeclaredAnnotations()

Constructor对象的方法

public T newInstance(Object ... initargs) //调用构造函数实例化对象  public int getModifiers(); //修饰符//声明在构造器上的注解@Target(ElementType.CONSTRUCTOR)<A extends Annotation> A getDeclaredAnnotation(Class<A> annotationClass)public Annotation[] getDeclaredAnnotations()

Method 对象的方法

public Object invoke(Object obj, Object... args) //反射执行方法，obj为对象，args为参数//如果是静态方法的话，obj为null, 不需要实例对象public int getModifiers() //修饰符//参数类型public Class<?>[] getParameterTypes() { return parameterTypes.clone(); }public int getParameterCount() { return parameterTypes.length; }//参数上的注解@Target(ElementType.PARAMETER)//因为一个方法上可能有多个参数，一个参数可能有多个注解，所以返回的是二维数组public Annotation[][] getParameterAnnotations()//获取方法上的注解@Target(ElementType.METHOD)<A extends Annotation> A getDeclaredAnnotation(Class<A> annotationClass);public Annotation[] getDeclaredAnnotations();

Modifier工具类

Java自带的类，可以用于判段某个类，方法，作用域的修饰符

public static boolean isPublic(int mod) { return (mod & PUBLIC) != 0; }public static boolean isPrivate(int mod) { return (mod & PRIVATE) != 0; }public static boolean isProtected(int mod) { return (mod & PROTECTED) != 0; }public static boolean isStatic(int mod) { return (mod & STATIC) != 0; }public static boolean isFinal(int mod) { return (mod & FINAL) != 0; }....public static final int PUBLIC           = 0x00000001;public static final int PRIVATE          = 0x00000002;public static final int PROTECTED        = 0x00000004;public static final int STATIC           = 0x00000008;public static final int FINAL            = 0x00000010;public static final int SYNCHRONIZED     = 0x00000020;public static final int VOLATILE         = 0x00000040;public static final int TRANSIENT        = 0x00000080;public static final int NATIVE           = 0x00000100;public static final int INTERFACE        = 0x00000200;public static final int ABSTRACT         = 0x00000400;public static final int STRICT           = 0x00000800;

注解Annotation

元注解

Java目前内置了四种元注解，负责注解其它的注解

@Target

表示该注解可以用在的地方

@Documented@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.ANNOTATION_TYPE)public @interface Target {    ElementType[] value();}

public enum ElementType {    //类，接口(包含注解类型) 或者枚举enum声明    TYPE,    //域声明，包含(enum实例)    FIELD,    //方法声明    METHOD,    //参数声明    PARAMETER,    //构造器声明    CONSTRUCTOR,    //局部变量声明    LOCAL_VARIABLE,    //注解类型    ANNOTATION_TYPE,    //包声明    PACKAGE,    /**     * Type parameter declaration     * @since 1.8     */    TYPE_PARAMETER,    /**     * Use of a type     * @since 1.8     */    TYPE_USE}

@Retention

注解信息的保留级别

可选的RetentionPolicy参数包括

SOURCE: 源代码中，编译时将被丢弃

CLASS:类文件中，在运行时会被VM丢弃

RUNTIME:运行时，也能够保留，可以通过反射机制读取注解的信息

public enum RetentionPolicy {    /**     * Annotations are to be discarded by the compiler.     */    SOURCE,    /**     * Annotations are to be recorded in the class file by the compiler     * but need not be retained by the VM at run time.  This is the default     * behavior.     */    CLASS,    /**     * Annotations are to be recorded in the class file by the compiler and     * retained by the VM at run time, so they may be read reflectively.     *     * @see java.lang.reflect.AnnotatedElement     */    RUNTIME}

@Documented

将注解包含在javadoc中

@Inherited

允许子类继承父类中的注解

一个简单的例子

如下面是一个简单的根据注解生成SQL语句的例子

@Target({ElementType.FIELD})@Retention(RetentionPolicy.RUNTIME)public @interface Column {    /**     * (可选) 列名     * Defaults to the field name     */    String name() default "";    /**     * (可选) 唯一性     */    boolean unique() default false;    /**     * （可选）能否为空     */    boolean nullable() default true;    /**     * 长度     */    int length() default 255;}

@Target(ElementType.FIELD)@Retention(RetentionPolicy.RUNTIME)public @interface Id {}

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)public @interface Table {    /**     * (可选) 表名     * Defaults to the entity name;     */    String name() default "";}

@Table(name="employee")public class Employee {    @Id    private Long id;    @Column(name="name", length = 20, nullable = false)    private String name;    @Column(name="address", unique = true)    private String address;    private Double salary;}

public class Main {    private static Map<Class, String> map;    static {        map = new HashMap<>();        map.put(byte.class, "TINYINT");        map.put(Byte.class, "TINYINT");        map.put(short.class, "INTEGER");        map.put(Short.class, "SMALLINT");        map.put(int.class, "INTEGER");        map.put(Integer.class, "INTEGER");        map.put(long.class, "BIGINT");        map.put(Long.class, "BIGINT");        map.put(char.class, "CHAR");        map.put(Character.class, "CHAR");        map.put(String.class, "VARCHAR");        map.put(double.class, "DOUBLE");        map.put(Double.class, "DOUBLE");        map.put(float.class, "FLOAT");        map.put(Float.class, "FLOAT");    }    public static void main(String[] args) {        System.out.println(new Main().sql(Employee.class));    }    private String sql(Class clazz) {        Table table = (Table)clazz.getDeclaredAnnotation(Table.class);        if(table == null) return "";        String tableName = table.name();        if(tableName.length() == 0)            tableName = clazz.getName();        Field[] fields = clazz.getDeclaredFields();        int fieldsLen = fields.length;        StringBuilder column_sql = new StringBuilder();        for(int i = 0; i < fieldsLen; i ++) {            Field currentField = fields[i];            String columnName = currentField.getName();            Column column = currentField.getAnnotation(Column.class);            if(column != null && column.name().length() > 0) {                columnName = column.name();            }            column_sql.append("\t").append(columnName).append(" ");            Class cla = currentField.getType();            column_sql.append(map.get(cla));            if(cla.equals(String.class))                column_sql.append("(").append(column == null ? 255 : column.length()).append(")");            column_sql.append(" ");            if(column != null) {                if(!column.nullable())                    column_sql.append("NOT NULL ");                if(column.unique())                    column_sql.append("UNIQUE ");            }            Annotation idAnt = currentField.getAnnotation(Id.class);            if(idAnt != null) {                column_sql.append("PRIMARY KEY ");            }            if( i + 1 != fieldsLen)                column_sql.append(",\n");        }        return "CREATE TABLE " + tableName + "(\n" + column_sql.toString() +"\n);";    }}

//程序输出

CREATE TABLE employee(        id BIGINT PRIMARY KEY ,        name VARCHAR(20) NOT NULL ,        address VARCHAR(255) UNIQUE ,        salary DOUBLE);

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