Assert(斷言)的初步理解構思
Web 應用在接受表單提交的數據后都需要對其進行合法性檢查，如果表單數據不合法，請求將被駁回。類似的，當我們在編寫類的方法時，也常常需要對方法入參進行合 法性檢查，如果入參不符合要求，方法將通過拋出異常的方式拒絕后續處理。舉一個例子：有一個根據文件名獲取輸入流的方法：InputStream getData(String file)，為了使方法能夠成功執行，必須保證 file 入參不能為 null 或空白字符，否則根本無須進行后繼的處理。這時方法的編寫者通常會在方法體的最前面編寫一段對入參進行檢測的代碼，如下所示：
public InputStream getData(String file) { if (file == null || file.length() == 0|| file.replaceAll("\\s", "").length() == 0) { throw new IllegalArgumentException("file入參不是有效的文件地址"); } … }
類似以上檢測方法入參的代碼是非常常見，但是在每個方法中都使用手工編寫檢測邏輯的方式並不是一個好主意。閱讀 Spring 源碼，您會發現 Spring 采用一個 org.springframework.util.Assert 通用類完成這一任務。
Assert 翻譯為中文為“斷言”，使用過 JUnit 的讀者都熟知這個概念，它斷定某一個實際的運行值和預期想一樣，否則就拋出異常。Spring 對方法入參的檢測借用了這個概念，其提供的 Assert 類擁有眾多按規則對方法入參進行斷言的方法，可以滿足大部分方法入參檢測的要求。這些斷言方法在入參不滿足要求時就會拋出 IllegalArgumentException。
Assert 類中的常用斷言方法:
1. notNull(Object object) 當 object 不為 null 時拋出異常，notNull(Object object, String message) 方法允許您通過 message 定制異常信息。和 notNull() 方法斷言規則相反的方法是 isNull(Object object)/isNull(Object object, String message)，它要求入參一定是 null； 2. isTrue(boolean expression) / isTrue(boolean expression, String message) 當 expression 不為 true 拋出異常； 3. notEmpty(Collection collection) / notEmpty(Collection collection, String message) 當集合未包含元素時拋出異常。 notEmpty(Map map) / notEmpty(Map map, String message) 和 notEmpty(Object[] array, String message) / notEmpty(Object[] array, String message) 分別對 Map 和 Object[] 類型的入參進行判斷； 4. hasLength(String text) / hasLength(String text, String message)
當 text 為 null 或長度為 0 時拋出異常； 5. hasText(String text) / hasText(String text, String message)
text 不能為 null 且必須至少包含一個非空格的字符，否則拋出異常； 6. isInstanceOf(Class clazz, Object obj) / isInstanceOf(Class type, Object obj, String message)
如果 obj 不能被正確造型為 clazz 指定的類將拋出異常；
7. isAssignable(Class superType, Class subType) / isAssignable(Class superType, Class subType, String message)
subType 必須可以按類型匹配於 superType，否則將拋出異常； 使用 Assert 斷言類可以簡化方法入參檢測的代碼，如 InputStream getData(String file) 在應用 Assert 斷言類后，其代碼可以簡化為以下的形式： public InputStream getData(String file){ Assert.hasText(file,"file入參不是有效的文件地址"); ① 使用 Spring 斷言類進行方法入參檢測 … }
可見使用 Spring 的 Assert 替代自編碼實現的入參檢測邏輯后，方法的簡潔性得到了不少的提高。Assert 不依賴於 Spring 容器，您可以大膽地在自己的應用中使用這個工具類.
源碼：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"); }