Understanding the Length Limits of Java String: JVM Specification and Practical Limits

Many developers wonder whether Java's String has a length limit; the author encountered this question in interviews and in a real project where a large file was Base64‑encoded into a string. This article examines the specifications and practical limits.

String stores its characters in a char[] array, so understanding array limits is the first step.

In Java, array indices are of type int . The maximum value of an int is 2^31‑1 (2,147,483,647), which means a char[] can theoretically hold up to about 2.1 billion characters, roughly 4 GB of data.

int[] arr1 = new int[10]; // define an array of length 10
int[] arr2 = {1,2,3,4,5}; // array length is 5

However, when a String literal is compiled, it is stored in the class file's constant pool. The JVM specification defines the constant‑pool entry for a String as CONSTANT_String , which references a CONSTANT_Utf8_info structure. The length field of this structure is a u2 (unsigned 2‑byte) value, limiting the stored UTF‑8 bytes to 2^16‑1 = 65535 . Because the JVM needs one extra byte for the terminating instruction, the effective maximum length for a literal string is 65534 characters.

To verify this limit, the author built a string of length 65,534 using a loop, confirmed the length with an online tool, and then assigned it as a literal in source code. The compilation succeeded, demonstrating that the compile‑time limit is indeed 65,534 characters.

In summary, at runtime a String can be as large as the maximum int value (≈4 GB), but when defined as a literal the JVM constant‑pool restricts it to 65,534 characters (effectively 65,534 due to the required terminating byte). Exceeding this limit causes a compile‑time error, while concatenated or dynamically created strings can exceed it.

The article concludes by inviting readers to like, comment, and share their thoughts.