第 39 条：注解优于命名模式

注解优于命名模式.jpeg

示例1

// Marker annotation type declaration

import java.lang.annotation.*;
/**

 * Indicates that the annotated method is a test method.

 * Use only on parameterless static methods.

 */

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

}

// Program containing marker annotations

public class Sample {
    @Test public static void m1() { }  // Test should pass
    public static void m2() { }
    @Test public static void m3() {     // Test should fail
        throw new RuntimeException("Boom");
    }
    public static void m4() { }
    @Test public void m5() { } // INVALID USE: nonstatic method
    public static void m6() { }
    @Test public static void m7() {    // Test should fail
        throw new RuntimeException("Crash");
    }
    public static void m8() { }
}

// Program to process marker annotations

import java.lang.reflect.*;

public class RunTests {
    public static void main(String[] args) throws Exception {
        int tests = 0;
        int passed = 0;
        Class<?> testClass = Class.forName(args[0]);
        for (Method m : testClass.getDeclaredMethods()) {
            if (m.isAnnotationPresent(Test.class)) {
                tests++;
                try {
                    m.invoke(null);
                    passed++;
                } catch (InvocationTargetException wrappedExc) {
                    Throwable exc = wrappedExc.getCause();
                    System.out.println(m + " failed: " + exc);
                } catch (Exception exc) {
                    System.out.println("Invalid @Test: " + m);
                }
            }
        }
        System.out.printf("Passed: %d, Failed: %d%n",
                          passed, tests - passed);
    }
}

public static void Sample.m3() failed: RuntimeException: Boom
Invalid @Test: public void Sample.m5()
public static void Sample.m7() failed: RuntimeException: Crash
Passed: 1, Failed: 3

示例2

// Annotation type with a parameter

import java.lang.annotation.*;

/**
 * Indicates that the annotated method is a test method that
 * must throw the designated exception to succeed.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface ExceptionTest {
    Class<? extends Throwable> value();
}

// Program containing annotations with a parameter

public class Sample2 {
    @ExceptionTest(ArithmeticException.class)
    public static void m1() {  // Test should pass
        int i = 0;
        i = i / i;
    }

    @ExceptionTest(ArithmeticException.class)
    public static void m2() {  // Should fail (wrong exception)
        int[] a = new int[0];
        int i = a[1];
    }

    @ExceptionTest(ArithmeticException.class)
    public static void m3() { }  // Should fail (no exception)
}

if (m.isAnnotationPresent(ExceptionTest.class)) {
    tests++;
    try {
        m.invoke(null);
        System.out.printf("Test %s failed: no exception%n", m);
    } catch (InvocationTargetException wrappedEx) {
        Throwable exc = wrappedEx.getCause();
        Class<? extends Throwable> excType =
            m.getAnnotation(ExceptionTest.class).value();
        if (excType.isInstance(exc)) {
            passed++;
        } else {
            System.out.printf(
                "Test %s failed: expected %s, got %s%n",
                m, excType.getName(), exc);
        }
    } catch (Exception exc) {
        System.out.println("Invalid @Test: " + m);
    }
}