我知道每个对象都需要堆内存，堆栈上的每个原语/引用都需要堆栈内存。

当我试图在堆上创建一个对象，而内存不足时，JVM会在堆上创建一个java.lang.OutOfMemoryError，并将它抛出给我。

因此，这意味着JVM在启动时保留了一些内存。

当这个保留内存用完(它肯定会用完，请阅读下面的讨论)并且JVM在堆上没有足够的内存来创建java.lang.OutOfMemoryError实例时会发生什么?

它只是挂着吗?或者他会给我一个空，因为没有内存新的OOM实例?

try {
    Object o = new Object();
    // and operations which require memory (well.. that's like everything)
} catch (java.lang.OutOfMemoryError e) {
    // JVM had insufficient memory to create an instance of java.lang.OutOfMemoryError to throw to us
    // what next? hangs here, stuck forever?
    // or would the machine decide to throw us a "null" ? (since it doesn't have memory to throw us anything more useful than a null)
    e.printStackTrace(); // e.printStackTrace() requires memory too.. =X
}

==

为什么JVM不能预留足够的内存?

无论保留了多少内存，如果JVM没有办法“回收”内存，仍然有可能用完内存:

try {
    Object o = new Object();
} catch (java.lang.OutOfMemoryError e) {
    // JVM had 100 units of "spare memory". 1 is used to create this OOM.
    try {
        e.printStackTrace();
    } catch (java.lang.OutOfMemoryError e2) {
        // JVM had 99 units of "spare memory". 1 is used to create this OOM.
        try {
            e.printStackTrace();
        } catch (java.lang.OutOfMemoryError e3) {
            // JVM had 98 units of "spare memory". 1 is used to create this OOM.
            try {
                e.printStackTrace();
            } catch (java.lang.OutOfMemoryError e4) {
                // JVM had 97 units of "spare memory". 1 is used to create this OOM.
                try {
                    e.printStackTrace();
                } catch (java.lang.OutOfMemoryError e5) {
                    // JVM had 96 units of "spare memory". 1 is used to create this OOM.
                    try {
                        e.printStackTrace();
                    } catch (java.lang.OutOfMemoryError e6) {
                        // JVM had 95 units of "spare memory". 1 is used to create this OOM.
                        e.printStackTrace();
                        //........the JVM can't have infinite reserved memory, he's going to run out in the end
                    }
                }
            }
        }
    }
}

或者更简洁地说:

private void OnOOM(java.lang.OutOfMemoryError e) {
    try {
        e.printStackTrace();
    } catch (java.lang.OutOfMemoryError e2) {
        OnOOM(e2);
    }
}