Are there inline functions in java?

Question

In Java, the optimizations are usually done at the JVM level. At runtime, the JVM perform some "complicated" analysis to determine which methods to inline. It can be aggressive in inlining, and the Hotspot JVM actually can inline non-final methods.

The java compilers almost never inline any method call (the JVM does all of that at runtime). They do inline compile time constants (e.g. final static primitive values). But not methods.

For more resources:

1. Article: The Java HotSpot Performance Engine: Method Inlining Example

2. Wiki: Inlining in OpenJDK, not fully populated but contains links to useful discussions.

No, there is no inline function in java. Yes, you can use a public static method anywhere in the code when placed in a public class. The java compiler may do inline expansion on a static or final method, but that is not guaranteed.

Typically such code optimizations are done by the compiler in combination with the JVM/JIT/HotSpot for code segments used very often. Also other optimization concepts like register declaration of parameters are not known in java.

Optimizations cannot be forced by declaration in java, but done by compiler and JIT. In many other languages these declarations are often only compiler hints (you can declare more register parameters than the processor has, the rest is ignored).

Declaring java methods static, final or private are also hints for the compiler. You should use it, but no garantees. Java performance is dynamic, not static. First call to a system is always slow because of class loading. Next calls are faster, but depending on memory and runtime the most common calls are optimized withinthe running system, so a server may become faster during runtime!

Java does not provide a way to manually suggest that a method should be inlined. As @notnoop says in the comments, the inlining is typically done by the JVM at execution time.

What you said above is correct. Sometimes final methods are created as inline, but there is no other way to explicitly create an inline function in java.

Well, there are methods could be called "inline" methods in java, but depending on the jvm. After compiling, if the method's machine code is less than 35 byte, it will be transferred to a inline method right away, if the method's machine code is less than 325 byte, it could be transferred into a inline method, depending on the jvm.

Real life example:

public class Control {
    public static final long EXPIRED_ON = 1386082988202l;
    public static final boolean isExpired() {
        return (System.currentTimeMillis() > EXPIRED_ON);
    }
}

Then in other classes, I can exit if the code has expired. If I reference the EXPIRED_ON variable from another class, the constant is inline to the byte code, making it very hard to track down all places in the code that checks the expiry date. However, if the other classes invoke the isExpired() method, the actual method is called, meaning a hacker could replace the isExpired method with another which always returns false.

I agree it would be very nice to force a compiler to inline the static final method to all classes which reference it. In that case, you need not even include the Control class, as it would not be needed at runtime.

From my research, this cannot be done. Perhaps some Obfuscator tools can do this, or, you could modify your build process to edit sources before compile.

As for proving if the method from the control class is placed inline to another class during compile, try running the other class without the Control class in the classpath.