reflect

java反射案例

一、简单注解案例

1.1 编写注解

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Test {
}

1.2 注入注解

public class Testable {
    public void execute() {
        System.out.println("executing...");
    }

    @Test
    void testExecute() {
        execute();
    }
}

1.3 编写注解处理器

public class TestTracker {
    public static void invoke(String[] classes) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
        if (!(classes.length > 0)) {
            System.out.println("classes为空！");
            System.exit(0);
        }
        for (String className : classes) {
            Class<?> clazz = Class.forName(className);
            if (clazz == null) {
                continue;
            }
            Method[] methods = clazz.getDeclaredMethods();
            for (Method method : methods) {
                Annotation[] anns = method.getDeclaredAnnotations();
                if (anns.length < 1) {
                    continue;
                }
                for (Annotation ann : anns) {
                    if (ann instanceof Test) {
                        Constructor<?> constructor = clazz.getConstructor();
                        Testable o = (Testable)constructor.newInstance();
                        method.setAccessible(true);
                        System.out.println(method.getName());
                        method.invoke(o);
                    }
                }
            }
        }
    }
}

1.4 运行测试

public class Main {
    public static void main(String[] args) {
        testTestTracker();
    }

    static void testTestTracker(){
        String[] tests = new String[]{"com.bamboo.demo.Testable"};
        try {
            TestTracker.invoke(tests);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

1.5 测试结果

testExecute
executing...

二、SQL注解案例

2.1 编写注解

2.1.1 Constraints注解

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Constraints {
    boolean primaryKey() default false;

    boolean allowNull() default true;

    boolean unique() default false;
}

2.1.2 SQLInteger注解

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLInteger {
    String name() default "";
    Constraints constraints() default @Constraints;
}

2.1.3 SQLString注解

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLString {
    int value() default 0;
    String name() default "";
    Constraints constraints() default @Constraints;
}

2.1.4 Uniquenss注解

public @interface Uniquenss {
    Constraints constraints() default @Constraints(unique = true);
}

2.2 注入注解

@DBTable(name = "MEMBER")
public class Member {
    @SQLString(30)
    private String firstName;
    @SQLString(50)
    private String lastName;
    @SQLInteger
    private Integer age;
    @SQLString(value = 30,constraints = @Constraints(primaryKey = true))
    private String handle;
    static int memberCount;
    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public Integer getAge() {
        return age;
    }

    public String getHandle() {
        return handle;
    }

    @Override
    public String toString() {
        return handle;
    }
}

2.3 编写注解处理器

public class TableCreator {
    public static String create(String[] classes) throws ClassNotFoundException {
        if (classes.length < 1) {
            System.out.println("classes: annotated classes");
            System.exit(0);
        }
        // 遍历每一个table表单Class
        for (String className : classes) {
            // 获取表单的反射类
            Class<?> cl = Class.forName(className);
            // 获取表单名注解
            DBTable dbTable = cl.getDeclaredAnnotation(DBTable.class);
            // 没有获取到说明这个类不是表单，进入下一次循环
            if (dbTable == null) {
                System.out.println("No DBTable annotations in class " + className);
                continue;
            }
            // 获取表单注解的名字为表名
            String tableName = dbTable.name();
            if (tableName.isEmpty()) {
                tableName = cl.getName().toUpperCase();
            }
            // 创建一个集合来存储列代码
            List<String> columnDefs = new ArrayList<>();
            // 通过获取反射类所有的域来进行查找域变量注解
            for (Field field : cl.getDeclaredFields()) {
                // 这是一个通用的列名，获取的列名都保存在这里
                @SuppressWarnings("all")
                String columnName = null;
                // 获取所有域的注解
                Annotation[] anns = field.getDeclaredAnnotations();
                // 当域注解长度不足 1 时，说明这个域没有标识注解，进行下一个域判断
                if (anns.length < 1) {
                    continue; // not a db table column
                }
                // 遍历每个注解直至找到SQL相关注解
                for (Annotation ann : anns) {
                    // SQLInteger判断
                    if (ann instanceof SQLInteger) {
                        SQLInteger sInt = (SQLInteger) anns[0];
                        // use field name if name not specified
                        if (sInt.name().isEmpty()) {
                            columnName = field.getName().toUpperCase();
                        } else {
                            columnName = sInt.name();
                        }
                        columnDefs.add(columnName + " INT" +
                                getConstraints(sInt.constraints())
                        );
                        break;
                    }
                    // SQLString判断
                    if (ann instanceof SQLString) {
                        SQLString sString = (SQLString) anns[0];
                        // use field name if name not specified
                        if (sString.name().isEmpty()) {
                            columnName = field.getName().toUpperCase();
                        } else {
                            columnName = sString.name();
                        }
                        columnDefs.add(columnName + " VARCHAR(" + sString.value() + ")" +
                                getConstraints(sString.constraints())
                        );
                        break;
                    }
                }
            }
            StringBuilder createCommand = new StringBuilder("CREATE TABLE " + tableName + "(");
            for (String columnDef : columnDefs) {
                createCommand.append("\n    " + columnDef + ",");
            }
            // remove trailing comma
            String tableCreate = createCommand.substring(0, createCommand.length() - 1) + ");";
            return tableCreate;
        }
        return null;
    }

    private static String getConstraints(Constraints con) {
        StringBuilder constraints = new StringBuilder();
        if (!con.allowNull()) {
            constraints.append(" NOT NULL");
        }
        if (con.primaryKey()) {
            constraints.append(" PRIMARY KEY");
        }
        if (con.unique()) {
            constraints.append(" UNIQUE");
        }
        return constraints.toString();
    }
}

2.4 运行测试

public class Main {
    public static void main(String[] args) {
        testCreateTable();
    }

    static void testCreateTable() {
        try {
            String code = TableCreator.create(new String[]{"com.bamboo.demo.Member"});
            System.out.println(code);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException(e);
        }
    }
}

2.5 测试结果

CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30),
    LASTNAME VARCHAR(50),
    AGE INT,
    HANDLE VARCHAR(30) PRIMARY KEY);