【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 
通常service每个公开的方法，都要先验证传入的参数是否合法，如果参数的值不合法直接抛异常。举一个简单的例子：有一个根据username获取User的方法 getUser(String username);
为了安全性，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：
Java代码 
public User getUser(String username) {
if (username == null || username.trim().length() != 0)
throw new IllegalArgumentException("username is invalid,invalid username:" + username);
//.......省去
}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,代码也不优雅！！！！
spring提供了org.springframework.util.Assert通用类。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
使用 Assert 断言类可以简化方法入参检测的代码，如 getUser(String username) 在应用 Assert 断言类后，其代码可以简化为以下的形式：
Java代码
public int getUser(String username) {
Assert.hasText(username);//这里没有使用自定义的错误描述，而是使用默认的错误描述，
}
spring参数验证的断言跟jdk的断言没有关系！
看下源代码就明白了。spring的断言是如何实现的！
Ａｓｓｅｒｔ.java
Java代码
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");
}来源：oschina
链接：https://my.oschina.net/u/2658078/blog/644845