Assert(断言)的初步理解构思
Web 应用在接受表单提交的数据后都需要对其进行合法性检查，如果表单数据不合法，请求将被驳回。类似的，当我们在编写类的方法时，也常常需要对方法入参进行合 法性检查，如果入参不符合要求，方法将通过抛出异常的方式拒绝后续处理。举一个例子：有一个根据文件名获取输入流的方法：InputStream getData(String file)，为了使方法能够成功执行，必须保证 file 入参不能为 null 或空白字符，否则根本无须进行后继的处理。这时方法的编写者通常会在方法体的最前面编写一段对入参进行检测的代码，如下所示：
public InputStream getData(String file) { if (file == null || file.length() == 0|| file.replaceAll("\s", "").length() == 0) { throw new IllegalArgumentException("file入参不是有效的文件地址"); } … }
类似以上检测方法入参的代码是非常常见，但是在每个方法中都使用手工编写检测逻辑的方式并不是一个好主意。阅读 Spring 源码，您会发现 Spring 采用一个 org.springframework.util.Assert 通用类完成这一任务。
Assert 翻译为中文为“断言”，使用过 JUnit 的读者都熟知这个概念，它断定某一个实际的运行值和预期想一样，否则就抛出异常。Spring 对方法入参的检测借用了这个概念，其提供的 Assert 类拥有众多按规则对方法入参进行断言的方法，可以满足大部分方法入参检测的要求。这些断言方法在入参不满足要求时就会抛出 IllegalArgumentException。
Assert 类中的常用断言方法:
1. notNull(Object object) 当 object 不为 null 时抛出异常，notNull(Object object, String message) 方法允许您通过 message 定制异常信息。和 notNull() 方法断言规则相反的方法是 isNull(Object object)/isNull(Object object, String message)，它要求入参一定是 null； 2. isTrue(boolean expression) / isTrue(boolean expression, String message) 当 expression 不为 true 抛出异常； 3. notEmpty(Collection collection) / notEmpty(Collection collection, String message) 当集合未包含元素时抛出异常。 notEmpty(Map map) / notEmpty(Map map, String message) 和 notEmpty(Object[] array, String message) / notEmpty(Object[] array, String message) 分别对 Map 和 Object[] 类型的入参进行判断； 4. hasLength(String text) / hasLength(String text, String message)
当 text 为 null 或长度为 0 时抛出异常； 5. hasText(String text) / hasText(String text, String message)
text 不能为 null 且必须至少包含一个非空格的字符，否则抛出异常； 6. isInstanceOf(Class clazz, Object obj) / isInstanceOf(Class type, Object obj, String message)
如果 obj 不能被正确造型为 clazz 指定的类将抛出异常；
7. isAssignable(Class superType, Class subType) / isAssignable(Class superType, Class subType, String message)
subType 必须可以按类型匹配于 superType，否则将抛出异常； 使用 Assert 断言类可以简化方法入参检测的代码，如 InputStream getData(String file) 在应用 Assert 断言类后，其代码可以简化为以下的形式： public InputStream getData(String file){ Assert.hasText(file,"file入参不是有效的文件地址"); ① 使用 Spring 断言类进行方法入参检测 … }
可见使用 Spring 的 Assert 替代自编码实现的入参检测逻辑后，方法的简洁性得到了不少的提高。Assert 不依赖于 Spring 容器，您可以大胆地在自己的应用中使用这个工具类.
源码：Assert.java
public abstract class Assert { /** * Assert a boolean expression, throwing <code>IllegalArgumentException</code> * if the test result is <code>false</code>. * <pre class="code">Assert.isTrue(i > 0, "The value must be greater than zero");</pre> * @param expression a boolean expression * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if expression is <code>false</code> */ public static void isTrue(boolean expression, String message) { if (!expression) { throw new IllegalArgumentException(message); } } /** * Assert a boolean expression, throwing <code>IllegalArgumentException</code> * if the test result is <code>false</code>. * <pre class="code">Assert.isTrue(i > 0);</pre> * @param expression a boolean expression * @throws IllegalArgumentException if expression is <code>false</code> */ public static void isTrue(boolean expression) { isTrue(expression, "[Assertion failed] - this expression must be true"); } /** * Assert that an object is <code>null</code> . * <pre class="code">Assert.isNull(value, "The value must be null");</pre> * @param object the object to check * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if the object is not <code>null</code> */ public static void isNull(Object object, String message) { if (object != null) { throw new IllegalArgumentException(message); } } /** * Assert that an object is <code>null</code> . * <pre class="code">Assert.isNull(value);</pre> * @param object the object to check * @throws IllegalArgumentException if the object is not <code>null</code> */ public static void isNull(Object object) { isNull(object, "[Assertion failed] - the object argument must be null"); } /** * Assert that an object is not <code>null</code> . * <pre class="code">Assert.notNull(clazz, "The class must not be null");</pre> * @param object the object to check * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if the object is <code>null</code> */ public static void notNull(Object object, String message) { if (object == null) { throw new IllegalArgumentException(message); } } /** * Assert that an object is not <code>null</code> . * <pre class="code">Assert.notNull(clazz);</pre> * @param object the object to check * @throws IllegalArgumentException if the object is <code>null</code> */ public static void notNull(Object object) { notNull(object, "[Assertion failed] - this argument is required; it must not be null"); } /** * Assert that the given String is not empty; that is, * it must not be <code>null</code> and not the empty String. * <pre class="code">Assert.hasLength(name, "Name must not be empty");</pre> * @param text the String to check * @param message the exception message to use if the assertion fails * @see StringUtils#hasLength */ public static void hasLength(String text, String message) { if (!StringUtils.hasLength(text)) { throw new IllegalArgumentException(message); } } /** * Assert that the given String is not empty; that is, * it must not be <code>null</code> and not the empty String. * <pre class="code">Assert.hasLength(name);</pre> * @param text the String to check * @see StringUtils#hasLength */ public static void hasLength(String text) { hasLength(text, "[Assertion failed] - this String argument must have length; it must not be null or empty"); } /** * Assert that the given String has valid text content; that is, it must not * be <code>null</code> and must contain at least one non-whitespace character. * <pre class="code">Assert.hasText(name, "'name' must not be empty");</pre> * @param text the String to check * @param message the exception message to use if the assertion fails * @see StringUtils#hasText */ public static void hasText(String text, String message) { if (!StringUtils.hasText(text)) { throw new IllegalArgumentException(message); } } /** * Assert that the given String has valid text content; that is, it must not * be <code>null</code> and must contain at least one non-whitespace character. * <pre class="code">Assert.hasText(name, "'name' must not be empty");</pre> * @param text the String to check * @see StringUtils#hasText */ public static void hasText(String text) { hasText(text, "[Assertion failed] - this String argument must have text; it must not be null, empty, or blank"); } /** * Assert that the given text does not contain the given substring. * <pre class="code">Assert.doesNotContain(name, "rod", "Name must not contain 'rod'");</pre> * @param textToSearch the text to search * @param substring the substring to find within the text * @param message the exception message to use if the assertion fails */ public static void doesNotContain(String textToSearch, String substring, String message) { if (StringUtils.hasLength(textToSearch) && StringUtils.hasLength(substring) && textToSearch.indexOf(substring) != -1) { throw new IllegalArgumentException(message); } } /** * Assert that the given text does not contain the given substring. * <pre class="code">Assert.doesNotContain(name, "rod");</pre> * @param textToSearch the text to search * @param substring the substring to find within the text */ public static void doesNotContain(String textToSearch, String substring) { doesNotContain(textToSearch, substring, "[Assertion failed] - this String argument must not contain the substring [" + substring + "]"); } /** * Assert that an array has elements; that is, it must not be * <code>null</code> and must have at least one element. * <pre class="code">Assert.notEmpty(array, "The array must have elements");</pre> * @param array the array to check * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if the object array is <code>null</code> or has no elements */ public static void notEmpty(Object[] array, String message) { if (ObjectUtils.isEmpty(array)) { throw new IllegalArgumentException(message); } } /** * Assert that an array has elements; that is, it must not be * <code>null</code> and must have at least one element. * <pre class="code">Assert.notEmpty(array);</pre> * @param array the array to check * @throws IllegalArgumentException if the object array is <code>null</code> or has no elements */ public static void notEmpty(Object[] array) { notEmpty(array, "[Assertion failed] - this array must not be empty: it must contain at least 1 element"); } /** * Assert that an array has no null elements. * Note: Does not complain if the array is empty! * <pre class="code">Assert.noNullElements(array, "The array must have non-null elements");</pre> * @param array the array to check * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if the object array contains a <code>null</code> element */ public static void noNullElements(Object[] array, String message) { if (array != null) { for (int i = 0; i < array.length; i++) { if (array[i] == null) { throw new IllegalArgumentException(message); } } } } /** * Assert that an array has no null elements. * Note: Does not complain if the array is empty! * <pre class="code">Assert.noNullElements(array);</pre> * @param array the array to check * @throws IllegalArgumentException if the object array contains a <code>null</code> element */ public static void noNullElements(Object[] array) { noNullElements(array, "[Assertion failed] - this array must not contain any null elements"); } /** * Assert that a collection has elements; that is, it must not be * <code>null</code> and must have at least one element. * <pre class="code">Assert.notEmpty(collection, "Collection must have elements");</pre> * @param collection the collection to check * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if the collection is <code>null</code> or has no elements */ public static void notEmpty(Collection collection, String message) { if (CollectionUtils.isEmpty(collection)) { throw new IllegalArgumentException(message); } } /** * Assert that a collection has elements; that is, it must not be * <code>null</code> and must have at least one element. * <pre class="code">Assert.notEmpty(collection, "Collection must have elements");</pre> * @param collection the collection to check * @throws IllegalArgumentException if the collection is <code>null</code> or has no elements */ public static void notEmpty(Collection collection) { notEmpty(collection, "[Assertion failed] - this collection must not be empty: it must contain at least 1 element"); } /** * Assert that a Map has entries; that is, it must not be <code>null</code> * and must have at least one entry. * <pre class="code">Assert.notEmpty(map, "Map must have entries");</pre> * @param map the map to check * @param message the exception message to use if the assertion fails * @throws IllegalArgumentException if the map is <code>null</code> or has no entries */ public static void notEmpty(Map map, String message) { if (CollectionUtils.isEmpty(map)) { throw new IllegalArgumentException(message); } } /** * Assert that a Map has entries; that is, it must not be <code>null</code> * and must have at least one entry. * <pre class="code">Assert.notEmpty(map);</pre> * @param map the map to check * @throws IllegalArgumentException if the map is <code>null</code> or has no entries */ public static void notEmpty(Map map) { notEmpty(map, "[Assertion failed] - this map must not be empty; it must contain at least one entry"); } /** * Assert that the provided object is an instance of the provided class. * <pre class="code">Assert.instanceOf(Foo.class, foo);</pre> * @param clazz the required class * @param obj the object to check * @throws IllegalArgumentException if the object is not an instance of clazz * @see Class#isInstance */ public static void isInstanceOf(Class clazz, Object obj) { isInstanceOf(clazz, obj, ""); } /** * Assert that the provided object is an instance of the provided class. * <pre class="code">Assert.instanceOf(Foo.class, foo);</pre> * @param type the type to check against * @param obj the object to check * @param message a message which will be prepended to the message produced by * the function itself, and which may be used to provide context. It should * normally end in a ": " or ". " so that the function generate message looks * ok when prepended to it. * @throws IllegalArgumentException if the object is not an instance of clazz * @see Class#isInstance */ public static void isInstanceOf(Class type, Object obj, String message) { notNull(type, "Type to check against must not be null"); if (!type.isInstance(obj)) { throw new IllegalArgumentException(message + "Object of class [" + (obj != null ? obj.getClass().getName() : "null") + "] must be an instance of " + type); } } /** * Assert that <code>superType.isAssignableFrom(subType)</code> is <code>true</code>. * <pre class="code">Assert.isAssignable(Number.class, myClass);</pre> * @param superType the super type to check * @param subType the sub type to check * @throws IllegalArgumentException if the classes are not assignable */ public static void isAssignable(Class superType, Class subType) { isAssignable(superType, subType, ""); } /** * Assert that <code>superType.isAssignableFrom(subType)</code> is <code>true</code>. * <pre class="code">Assert.isAssignable(Number.class, myClass);</pre> * @param superType the super type to check against * @param subType the sub type to check * @param message a message which will be prepended to the message produced by * the function itself, and which may be used to provide context. It should * normally end in a ": " or ". " so that the function generate message looks * ok when prepended to it. * @throws IllegalArgumentException if the classes are not assignable */ public static void isAssignable(Class superType, Class subType, String message) { notNull(superType, "Type to check against must not be null"); if (subType == null || !superType.isAssignableFrom(subType)) { throw new IllegalArgumentException(message + subType + " is not assignable to " + superType); } } /** * Assert a boolean expression, throwing <code>IllegalStateException</code> * if the test result is <code>false</code>. Call isTrue if you wish to * throw IllegalArgumentException on an assertion failure. * <pre class="code">Assert.state(id == null, "The id property must not already be initialized");</pre> * @param expression a boolean expression * @param message the exception message to use if the assertion fails * @throws IllegalStateException if expression is <code>false</code> */ public static void state(boolean expression, String message) { if (!expression) { throw new IllegalStateException(message); } } /** * Assert a boolean expression, throwing {@link IllegalStateException} * if the test result is <code>false</code>. * <p>Call {@link #isTrue(boolean)} if you wish to * throw {@link IllegalArgumentException} on an assertion failure. * <pre class="code">Assert.state(id == null);</pre> * @param expression a boolean expression * @throws IllegalStateException if the supplied expression is <code>false</code> */ public static void state(boolean expression) { state(expression, "[Assertion failed] - this state invariant must be true"); }