在 Java 中布尔变量的大小是多少？

It's virtual machine dependent.

The actual information represented by a boolean value in Java is one bit: 1 for true, 0 for false. However, the actual size of a boolean variable in memory is not precisely defined by the Java specification. See Primitive Data Types in Java.

The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.

The boolean values are compiled to int data type in JVM. See here.

It depends on the virtual machine, but it's easy to adapt the code from a similar question asking about bytes in Java:

class LotsOfBooleans
{
boolean a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;
boolean b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;
boolean c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;
boolean d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;
boolean e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;
}


class LotsOfInts
{
int a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;
int b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;
int c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;
int d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;
int e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;
}




public class Test
{
private static final int SIZE = 1000000;


public static void main(String[] args) throws Exception
{
LotsOfBooleans[] first = new LotsOfBooleans[SIZE];
LotsOfInts[] second = new LotsOfInts[SIZE];


System.gc();
long startMem = getMemory();


for (int i=0; i < SIZE; i++)
{
first[i] = new LotsOfBooleans();
}


System.gc();
long endMem = getMemory();


System.out.println ("Size for LotsOfBooleans: " + (endMem-startMem));
System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));


System.gc();
startMem = getMemory();
for (int i=0; i < SIZE; i++)
{
second[i] = new LotsOfInts();
}
System.gc();
endMem = getMemory();


System.out.println ("Size for LotsOfInts: " + (endMem-startMem));
System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));


// Make sure nothing gets collected
long total = 0;
for (int i=0; i < SIZE; i++)
{
total += (first[i].a0 ? 1 : 0) + second[i].a0;
}
System.out.println(total);
}


private static long getMemory()
{
Runtime runtime = Runtime.getRuntime();
return runtime.totalMemory() - runtime.freeMemory();
}
}

To reiterate, this is VM-dependent, but on my Windows laptop running Sun's JDK build 1.6.0_11 I got the following results:

Size for LotsOfBooleans: 87978576
Average size: 87.978576
Size for LotsOfInts: 328000000
Average size: 328.0

That suggests that booleans can basically be packed into a byte each by Sun's JVM.

It's undefined; doing things like Jon Skeet suggested will get you an approximation on a given platform, but the way to know precisely for a specific platform is to use a profiler.

On a side note...

If you are thinking about using an array of Boolean objects, don't. Use a BitSet instead - it has some performance optimisations (and some nice extra methods, allowing you to get the next set/unset bit).

I read that Java reserves one byte for a boolean datatype, but it uses only one bit. However, the documentation says that "its "size" isn't something that's precisely defined". See here.

Size of the boolean in java is virtual machine dependent. but Any Java object is aligned to an 8 bytes granularity. A Boolean has 8 bytes of header, plus 1 byte of payload, for a total of 9 bytes of information. The JVM then rounds it up to the next multiple of 8. so the one instance of java.lang.Boolean takes up 16 bytes of memory.