一、背景
今天无意间看到了java.util.concurrent.TimeUnit枚举类。
想通过这个类谈谈枚举，以及编程规范问题。
二、源码
源码：
代码语言：javascript复制package java.util.concurrent;
/**
* A {@code TimeUnit} represents time durations at a given unit of
* granularity and provides utility methods to convert across units,
* and to perform timing and delay operations in these units. A
* {@code TimeUnit} does not maintain time information, but only
* helps organize and use time representations that may be maintained
* separately across various contexts. A nanosecond is defined as one
* thousandth of a microsecond, a microsecond as one thousandth of a
* millisecond, a millisecond as one thousandth of a second, a minute
* as sixty seconds, an hour as sixty minutes, and a day as twenty four
* hours.
*
* A {@code TimeUnit} is mainly used to inform time-based methods
* how a given timing parameter should be interpreted. For example,
* the following code will timeout in 50 milliseconds if the {@link
* java.util.concurrent.locks.Lock lock} is not available:
*
* {@code
* Lock lock = ...;
* if (lock.tryLock(50L, TimeUnit.MILLISECONDS)) ...}
*
* while this code will timeout in 50 seconds:
* {@code
* Lock lock = ...;
* if (lock.tryLock(50L, TimeUnit.SECONDS)) ...}
*
* Note however, that there is no guarantee that a particular timeout
* implementation will be able to notice the passage of time at the
* same granularity as the given {@code TimeUnit}.
*
* @since 1.5
* @author Doug Lea
*/
public enum TimeUnit {
/**
* Time unit representing one thousandth of a microsecond
*/
NANOSECONDS {
public long toNanos(long d) { return d; }
public long toMicros(long d) { return d/(C1/C0); }
public long toMillis(long d) { return d/(C2/C0); }
public long toSeconds(long d) { return d/(C3/C0); }
public long toMinutes(long d) { return d/(C4/C0); }
public long toHours(long d) { return d/(C5/C0); }
public long toDays(long d) { return d/(C6/C0); }
public long convert(long d, TimeUnit u) { return u.toNanos(d); }
int excessNanos(long d, long m) { return (int)(d - (m*C2)); }
},
/**
* Time unit representing one thousandth of a millisecond
*/
MICROSECONDS {
public long toNanos(long d) { return x(d, C1/C0, MAX/(C1/C0)); }
public long toMicros(long d) { return d; }
public long toMillis(long d) { return d/(C2/C1); }
public long toSeconds(long d) { return d/(C3/C1); }
public long toMinutes(long d) { return d/(C4/C1); }
public long toHours(long d) { return d/(C5/C1); }
public long toDays(long d) { return d/(C6/C1); }
public long convert(long d, TimeUnit u) { return u.toMicros(d); }
int excessNanos(long d, long m) { return (int)((d*C1) - (m*C2)); }
},
/**
* Time unit representing one thousandth of a second
*/
MILLISECONDS {
public long toNanos(long d) { return x(d, C2/C0, MAX/(C2/C0)); }
public long toMicros(long d) { return x(d, C2/C1, MAX/(C2/C1)); }
public long toMillis(long d) { return d; }
public long toSeconds(long d) { return d/(C3/C2); }
public long toMinutes(long d) { return d/(C4/C2); }
public long toHours(long d) { return d/(C5/C2); }
public long toDays(long d) { return d/(C6/C2); }
public long convert(long d, TimeUnit u) { return u.toMillis(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing one second
*/
SECONDS {
public long toNanos(long d) { return x(d, C3/C0, MAX/(C3/C0)); }
public long toMicros(long d) { return x(d, C3/C1, MAX/(C3/C1)); }
public long toMillis(long d) { return x(d, C3/C2, MAX/(C3/C2)); }
public long toSeconds(long d) { return d; }
public long toMinutes(long d) { return d/(C4/C3); }
public long toHours(long d) { return d/(C5/C3); }
public long toDays(long d) { return d/(C6/C3); }
public long convert(long d, TimeUnit u) { return u.toSeconds(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing sixty seconds
*/
MINUTES {
public long toNanos(long d) { return x(d, C4/C0, MAX/(C4/C0)); }
public long toMicros(long d) { return x(d, C4/C1, MAX/(C4/C1)); }
public long toMillis(long d) { return x(d, C4/C2, MAX/(C4/C2)); }
public long toSeconds(long d) { return x(d, C4/C3, MAX/(C4/C3)); }
public long toMinutes(long d) { return d; }
public long toHours(long d) { return d/(C5/C4); }
public long toDays(long d) { return d/(C6/C4); }
public long convert(long d, TimeUnit u) { return u.toMinutes(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing sixty minutes
*/
HOURS {
public long toNanos(long d) { return x(d, C5/C0, MAX/(C5/C0)); }
public long toMicros(long d) { return x(d, C5/C1, MAX/(C5/C1)); }
public long toMillis(long d) { return x(d, C5/C2, MAX/(C5/C2)); }
public long toSeconds(long d) { return x(d, C5/C3, MAX/(C5/C3)); }
public long toMinutes(long d) { return x(d, C5/C4, MAX/(C5/C4)); }
public long toHours(long d) { return d; }
public long toDays(long d) { return d/(C6/C5); }
public long convert(long d, TimeUnit u) { return u.toHours(d); }
int excessNanos(long d, long m) { return 0; }
},
/**
* Time unit representing twenty four hours
*/
DAYS {
public long toNanos(long d) { return x(d, C6/C0, MAX/(C6/C0)); }
public long toMicros(long d) { return x(d, C6/C1, MAX/(C6/C1)); }
public long toMillis(long d) { return x(d, C6/C2, MAX/(C6/C2)); }
public long toSeconds(long d) { return x(d, C6/C3, MAX/(C6/C3)); }
public long toMinutes(long d) { return x(d, C6/C4, MAX/(C6/C4)); }
public long toHours(long d) { return x(d, C6/C5, MAX/(C6/C5)); }
public long toDays(long d) { return d; }
public long convert(long d, TimeUnit u) { return u.toDays(d); }
int excessNanos(long d, long m) { return 0; }
};
// Handy constants for conversion methods
static final long C0 = 1L;
static final long C1 = C0 * 1000L;
static final long C2 = C1 * 1000L;
static final long C3 = C2 * 1000L;
static final long C4 = C3 * 60L;
static final long C5 = C4 * 60L;
static final long C6 = C5 * 24L;
static final long MAX = Long.MAX_VALUE;
/**
* Scale d by m, checking for overflow.
* This has a short name to make above code more readable.
*/
static long x(long d, long m, long over) {
if (d > over) return Long.MAX_VALUE;
if (d < -over) return Long.MIN_VALUE;
return d * m;
}
// To maintain full signature compatibility with 1.5, and to improve the
// clarity of the generated javadoc (see 6287639: Abstract methods in
// enum classes should not be listed as abstract), method convert
// etc. are not declared abstract but otherwise act as abstract methods.
/**
* Converts the given time duration in the given unit to this unit.
* Conversions from finer to coarser granularities truncate, so
* lose precision. For example, converting {@code 999} milliseconds
* to seconds results in {@code 0}. Conversions from coarser to
* finer granularities with arguments that would numerically
* overflow saturate to {@code Long.MIN_VALUE} if negative or
* {@code Long.MAX_VALUE} if positive.
*
* For example, to convert 10 minutes to milliseconds, use:
* {@code TimeUnit.MILLISECONDS.convert(10L, TimeUnit.MINUTES)}
*
* @param sourceDuration the time duration in the given {@code sourceUnit}
* @param sourceUnit the unit of the {@code sourceDuration} argument
* @return the converted duration in this unit,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
*/
public long convert(long sourceDuration, TimeUnit sourceUnit) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) NANOSECONDS.convert(duration, this)}.
* @param duration the duration
* @return the converted duration,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
*/
public long toNanos(long duration) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) MICROSECONDS.convert(duration, this)}.
* @param duration the duration
* @return the converted duration,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
*/
public long toMicros(long duration) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) MILLISECONDS.convert(duration, this)}.
* @param duration the duration
* @return the converted duration,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
*/
public long toMillis(long duration) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) SECONDS.convert(duration, this)}.
* @param duration the duration
* @return the converted duration,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
*/
public long toSeconds(long duration) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) MINUTES.convert(duration, this)}.
* @param duration the duration
* @return the converted duration,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
* @since 1.6
*/
public long toMinutes(long duration) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) HOURS.convert(duration, this)}.
* @param duration the duration
* @return the converted duration,
* or {@code Long.MIN_VALUE} if conversion would negatively
* overflow, or {@code Long.MAX_VALUE} if it would positively overflow.
* @since 1.6
*/
public long toHours(long duration) {
throw new AbstractMethodError();
}
/**
* Equivalent to
* {@link #convert(long, TimeUnit) DAYS.convert(duration, this)}.
* @param duration the duration
* @return the converted duration
* @since 1.6
*/
public long toDays(long duration) {
throw new AbstractMethodError();
}
/**
* Utility to compute the excess-nanosecond argument to wait,
* sleep, join.
* @param d the duration
* @param m the number of milliseconds
* @return the number of nanoseconds
*/
abstract int excessNanos(long d, long m);
/**
* Performs a timed {@link Object#wait(long, int) Object.wait}
* using this time unit.
* This is a convenience method that converts timeout arguments
* into the form required by the {@code Object.wait} method.
*
* For example, you could implement a blocking {@code poll}
* method (see {@link BlockingQueue#poll BlockingQueue.poll})
* using:
*
* {@code
* public synchronized Object poll(long timeout, TimeUnit unit)
* throws InterruptedException {
* while (empty) {
* unit.timedWait(this, timeout);
* ...
* }
* }}
*
* @param obj the object to wait on
* @param timeout the maximum time to wait. If less than
* or equal to zero, do not wait at all.
* @throws InterruptedException if interrupted while waiting
*/
public void timedWait(Object obj, long timeout)
throws InterruptedException {
if (timeout > 0) {
long ms = toMillis(timeout);
int ns = excessNanos(timeout, ms);
obj.wait(ms, ns);
}
}
/**
* Performs a timed {@link Thread#join(long, int) Thread.join}
* using this time unit.
* This is a convenience method that converts time arguments into the
* form required by the {@code Thread.join} method.
*
* @param thread the thread to wait for
* @param timeout the maximum time to wait. If less than
* or equal to zero, do not wait at all.
* @throws InterruptedException if interrupted while waiting
*/
public void timedJoin(Thread thread, long timeout)
throws InterruptedException {
if (timeout > 0) {
long ms = toMillis(timeout);
int ns = excessNanos(timeout, ms);
thread.join(ms, ns);
}
}
/**
* Performs a {@link Thread#sleep(long, int) Thread.sleep} using
* this time unit.
* This is a convenience method that converts time arguments into the
* form required by the {@code Thread.sleep} method.
*
* @param timeout the minimum time to sleep. If less than
* or equal to zero, do not sleep at all.
* @throws InterruptedException if interrupted while sleeping
*/
public void sleep(long timeout) throws InterruptedException {
if (timeout > 0) {
long ms = toMillis(timeout);
int ns = excessNanos(timeout, ms);
Thread.sleep(ms, ns);
}
}
}我们发现 枚举其实可以定义普通方法的，每个枚举常量可以重写这些函数。
有些方法不重写会抛出抽象方法异常。
代码语言：javascript复制throw new AbstractMethodError()但是我们平时写业务代码的时候很少在枚举中定义逻辑方法，通常只是定义get方法，或者通过value获取枚举对象的方法。
另外我们通过源码可以看到，为什么TimeUnit.SECONDS.sleep(10);可以让当前线程暂停10秒呢？
代码语言：javascript复制 /**
* Performs a {@link Thread#sleep(long, int) Thread.sleep} using
* this time unit.
* This is a convenience method that converts time arguments into the
* form required by the {@code Thread.sleep} method.
*
* @param timeout the minimum time to sleep. If less than
* or equal to zero, do not sleep at all.
* @throws InterruptedException if interrupted while sleeping
*/
public void sleep(long timeout) throws InterruptedException {
if (timeout > 0) {
long ms = toMillis(timeout);
int ns = excessNanos(timeout, ms);
Thread.sleep(ms, ns);
}
}可以看到先调用枚举的转毫秒方法,然后获取纳秒。
代码语言：javascript复制/**
* Time unit representing one second
*/
SECONDS {
public long toNanos(long d) { return x(d, C3/C0, MAX/(C3/C0)); }
public long toMicros(long d) { return x(d, C3/C1, MAX/(C3/C1)); }
public long toMillis(long d) { return x(d, C3/C2, MAX/(C3/C2)); }
public long toSeconds(long d) { return d; }
public long toMinutes(long d) { return d/(C4/C3); }
public long toHours(long d) { return d/(C5/C3); }
public long toDays(long d) { return d/(C6/C3); }
public long convert(long d, TimeUnit u) { return u.toSeconds(d); }
int excessNanos(long d, long m) { return 0; }
},然后调用线程的sleep方法，分别本时间枚举传入毫秒和纳秒。
三、思考
该枚举存在一些问题，看下面这部分：
这里的变量是什么鬼？？这里的函数名是什么含义？？
能不能起点有意义的名字？
然后看这里：
既然不重写会抛出抽象方法错误，而且每个枚举对象都重写了这些方法，因此不如直接声明为抽象方法，让枚举对象强制重写不是更好吗？
四、总结
我们编写枚举类时，不要思维定式的只能添加值和字符串描述，可以根据业务需要编写一些和上述时间枚举类似的属于该枚举逻辑的函数。
另外建议属性和函数使用有意义的名称。
如果必须重写的函数，建议定义抽象函数或者实现接口，强制枚举对象实现。
