How to get all PsiClass inside a PsiMethod?

As the title says, I want to get all psiClass used in a psiMethod.

I've tried 2 ways but they don't work:

1. use psiMethod.getOwnReferences()

for (PsiSymbolReference psiSymbolReference: psiMethod.getOwnReferences()) {
    System.out.println(psiSymbolReference.getElement().getText());
}

The above code has no output, and I wonder why?

2. use complex way:

// get code block
PsiCodeBlock codeBlock = psiMethod.getBody();
if (codeBlock == null) {
    return psiClassList;
}
// get statements
PsiStatement[] statements = codeBlock.getStatements();
for (int index = 0; index < statements.length; ++index) {
    // whether it is a statement
    if (statements[index] instanceof PsiExpressionStatement) {
        PsiExpression psiExpression = ((PsiExpressionStatement) statements[index]).getExpression();
        /**
         * method call expression
         */
        if (psiExpression instanceof PsiMethodCallExpression) {
            ...
        }
        /**
         * new expression
         */
        if (psiExpression instanceof PsiNewExpression) {
            ...
        }
        /**
         * assignment expression
         */
        if (psiExpression instanceof PsiAssignmentExpression) {
           ...
        }
    }
}

It is kind of complex and I might not be able to cover all situations(for expamle, recurrence for code block statement?).

So I wonder whether there's a way to get all PsiClass references inside a psimethod