下面是我在Hibernate/SQL项目的服务器中编写的断言。一个实体bean有两个有效的布尔属性，称为isActive和isDefault。每个都可以有一个值“Y”或“N”或null，被视为“N”。我们希望确保浏览器客户端仅限于这三个值。所以，在这两个属性的setter中，我添加了这样的断言:

assert new HashSet<String>(Arrays.asList("Y", "N", null)).contains(value) : value;

注意以下几点。

This assertion is for the development phase only. If the client sends a bad value, we will catch that early and fix it, long before we reach production. Assertions are for defects that you can catch early. This assertion is slow and inefficient. That's okay. Assertions are free to be slow. We don't care because they're development-only tools. This won't slow down the production code because assertions will be disabled. (There's some disagreement on this point, which I'll get to later.) This leads to my next point. This assertion has no side effects. I could have tested my value against an unmodifiable static final Set, but that set would have stayed around in production, where it would never get used. This assertion exists to verify the proper operation of the client. So by the time we reach production, we will be sure that the client is operating properly, so we can safely turn the assertion off. Some people ask this: If the assertion isn't needed in production, why not just take them out when you're done? Because you'll still need them when you start working on the next version.

Some people have argued that you should never use assertions, because you can never be sure that all the bugs are gone, so you need to keep them around even in production. And so there's no point in using the assert statement, since the only advantage to asserts is that you can turn them off. Hence, according to this thinking, you should (almost) never use asserts. I disagree. It's certainly true that if a test belongs in production, you should not use an assert. But this test does not belong in production. This one is for catching a bug that's not likely to ever reach production, so it may safely be turned off when you're done.

顺便说一句，我可以这样写:

assert value == null || value.equals("Y") || value.equals("N") : value;

对于只有三个值是可以的，但是如果可能值的数量变大，HashSet版本就会变得更方便。我选择HashSet版本来说明我关于效率的观点。

默认情况下，断言是禁用的。要启用它们，我们必须使用-ea选项运行程序(粒度可以改变)。例如，java -ea AssertionsDemo。

使用断言有两种格式:

简单:如。维护1 = = 2;//这将引发AssertionError。 更好的方法:断言1==2:“不可能..1不等于2”; 这将引发AssertionError，并显示给定的消息，因此更好。虽然实际的语法是断言expr1:expr2，其中expr2可以是返回值的任何表达式，但我更多地使用它只是为了打印消息。

基本上，“断言为真”会通过，“断言为假”会失败。让我们看看这是如何工作的:

public static void main(String[] args)
{
    String s1 = "Hello";
    assert checkInteger(s1);
}

private static boolean checkInteger(String s)
{
    try {
        Integer.parseInt(s);
        return true;
    }
    catch(Exception e)
    {
        return false;
    }
}

在Java中assert关键字是做什么的?

让我们看看编译后的字节码。

我们将得出结论:

public class Assert {
    public static void main(String[] args) {
        assert System.currentTimeMillis() == 0L;
    }
}

生成几乎完全相同的字节码:

public class Assert {
    static final boolean $assertionsDisabled =
        !Assert.class.desiredAssertionStatus();
    public static void main(String[] args) {
        if (!$assertionsDisabled) {
            if (System.currentTimeMillis() != 0L) {
                throw new AssertionError();
            }
        }
    }
}

其中Assert.class.desiredAssertionStatus()在命令行传递-ea时为true，否则为false。

我们使用System.currentTimeMillis()来确保它不会被优化掉(assert true;所做的那样)。

合成字段生成后，Java只需要在加载时调用Assert.class.desiredAssertionStatus()一次，然后将结果缓存到那里。参见:“静态合成”是什么意思?

我们可以用以下方法验证:

javac Assert.java
javap -c -constants -private -verbose Assert.class

在Oracle JDK 1.8.0_45中，生成了一个合成的静态字段(参见:“静态合成”是什么意思?):

static final boolean $assertionsDisabled;
  descriptor: Z
  flags: ACC_STATIC, ACC_FINAL, ACC_SYNTHETIC

与静态初始化项一起使用:

 0: ldc           #6                  // class Assert
 2: invokevirtual #7                  // Method java/lang Class.desiredAssertionStatus:()Z
 5: ifne          12
 8: iconst_1
 9: goto          13
12: iconst_0
13: putstatic     #2                  // Field $assertionsDisabled:Z
16: return

主要方法是:

 0: getstatic     #2                  // Field $assertionsDisabled:Z
 3: ifne          22
 6: invokestatic  #3                  // Method java/lang/System.currentTimeMillis:()J
 9: lconst_0
10: lcmp
11: ifeq          22
14: new           #4                  // class java/lang/AssertionError
17: dup
18: invokespecial #5                  // Method java/lang/AssertionError."<init>":()V
21: athrow
22: return

我们的结论是:

assert没有字节码级别的支持:它是Java语言的概念 assert可以用系统属性-Pcom.me很好地模拟。assert=true替换命令行上的-ea，并抛出新的AssertionError()。

下面是最常见的用例。假设你正在打开一个枚举值:

switch (fruit) {
  case apple:
    // do something
    break;
  case pear:
    // do something
    break;
  case banana:
    // do something
    break;
}

只要你处理每一个案子，就没问题。但是有一天，有人会把fig添加到你的枚举中，而忘记把它添加到你的switch语句中。这将产生一个难以捕捉的错误，因为直到您离开switch语句后才会感受到其影响。但是如果你像这样写开关，你可以立即捕获它:

switch (fruit) {
  case apple:
    // do something
    break;
  case pear:
    // do something
    break;
  case banana:
    // do something
    break;
  default:
    assert false : "Missing enum value: " + fruit;
}

一个真实的例子，来自一个stack类(来自Java文章中的断言)

public int pop() {
   // precondition
   assert !isEmpty() : "Stack is empty";
   return stack[--num];
}