小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
在代码中 每次编译,经常会有很多警告信息, 影响对代码错误信息的过滤,使用@SuppressWarnings注解,可以消除指定的编译器警告问题
1 @SuppressWarnings的简介
@SuppressWarnings的作用是让编译器不产生警告信息. 从源码可知,该注解从JDK1.5版本开始使用,它支持在类,属性,方法,参数,构造,本地变量上使用.注解中只有一个变量value, 字符串数组类型,支持多个类型的警告.
/**
* Indicates that the named compiler warnings should be suppressed in the
* annotated element (and in all program elements contained in the annotated
* element). Note that the set of warnings suppressed in a given element is
* a superset of the warnings suppressed in all containing elements. For
* example, if you annotate a class to suppress one warning and annotate a
* method to suppress another, both warnings will be suppressed in the method.
*
* <p>As a matter of style, programmers should always use this annotation
* on the most deeply nested element where it is effective. If you want to
* suppress a warning in a particular method, you should annotate that
* method rather than its class.
*
* @author Josh Bloch
* @since 1.5
* @jls 4.8 Raw Types
* @jls 4.12.2 Variables of Reference Type
* @jls 5.1.9 Unchecked Conversion
* @jls 5.5.2 Checked Casts and Unchecked Casts
* @jls 9.6.3.5 @SuppressWarnings
*/
// 类, 字段, 方法, 函数入参, 构造,局部变量
@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
/**
* The set of warnings that are to be suppressed by the compiler in the
* annotated element. Duplicate names are permitted. The second and
* successive occurrences of a name are ignored. The presence of
* unrecognized warning names is <i>not</i> an error: Compilers must
* ignore any warning names they do not recognize. They are, however,
* free to emit a warning if an annotation contains an unrecognized
* warning name.
*
* <p> The string {@code "unchecked"} is used to suppress
* unchecked warnings. Compiler vendors should document the
* additional warning names they support in conjunction with this
* annotation type. They are encouraged to cooperate to ensure
* that the same names work across multiple compilers.
* @return the set of warnings to be suppressed
*/
String[] value();
}
常见使用的类型:
- 单类型警告 如: @SuppressWarnings("unchecked") 抑制没有进行类型检查操作的警告
- 多类型警告 如: @SuppressWarnings("unchecked","unused") 抑制没有进行类型检查操作和没被使用过的代码的警告
- 所有类型警告 如: @SuppressWarnings("all") 抑制所有警告
警告的关键字清单:
| 关键字 | 说明 |
|---|---|
| all | to suppress all warnings （抑制所有警告） |
| boxing | to suppress warnings relative to boxing/unboxing operations （抑制装箱、拆箱操作时候的警告） |
| cast | to suppress warnings relative to cast operations （抑制映射相关的警告） |
| dep-ann | to suppress warnings relative to deprecated annotation （抑制启用注释的警告） |
| deprecation | to suppress warnings relative to deprecation （抑制过期方法警告） |
| fallthrough | to suppress warnings relative to missing breaks in switch statements （抑制确在switch中缺失breaks的警告） |
| finally | to suppress warnings relative to finally block that don’t return （抑制finally模块没有返回的警告） |
| hiding | to suppress warnings relative to locals that hide variable（抑制相对于隐藏变量的局部变量的警告） |
| incomplete-switch | to suppress warnings relative to missing entries in a switch statement (enum case)（忽略没有完整的switch语句） |
| nls | to suppress warnings relative to non-nls string literals（ 忽略非nls格式的字符） |
| null | to suppress warnings relative to null analysis（ 忽略对null的操作） |
| rawtypes | to suppress warnings relative to un-specific types when using generics on class params（ 使用generics时忽略没有指定相应的类型,即没有传递带有泛型的参数） |
| restriction | to suppress warnings relative to usage of discouraged or forbidden references（ 抑制禁止使用劝阻或禁止引用的警告） |
| serial | to suppress warnings relative to missing serialVersionUID field for a serializable class（ 忽略在serializable类中没有声明serialVersionUID变量） |
| static-access | to suppress warnings relative to incorrect static access（ 抑制不正确的静态访问方式警告） |
| synthetic-access | to suppress warnings relative to unoptimized access from inner classes（ 抑制子类没有按最优方法访问内部类的警告） |
| unchecked | to suppress warnings relative to unchecked operations（ 抑制没有进行类型检查操作的警告） |
| unqualified-field-access | to suppress warnings relative to field access unqualified（ 抑制没有权限访问的域的警告） |
| unused | to suppress warnings relative to unused code（ 抑制没被使用过的代码的警告） |
2 @SuppressWarnings的使用
// 抑制未使用警告
@SuppressWarnings("unused")
public void test1(){
try {
// 未使用警告
String s = "";
} catch (Exception e) {
e.printStackTrace();
}
}
// 抑制未进行类型检查操作警告 未指定相应的类型警告 未使用警告
@SuppressWarnings({"unchecked","rawtypes","unused"})
public void test2(){
try {
// 未使用警告
String s = "";
// 未进行类型检查操作警告 未指定相应的类型警告
List items = new ArrayList();
items.add("李白");
} catch (Exception e) {
e.printStackTrace();
}
}
// 抑制所有警告
@SuppressWarnings({"all"})
public void test3(){
try {
String s = "";
List items = new ArrayList();
items.add("李白");
} catch (Exception e) {
e.printStackTrace();
}
}
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